Prenatal or antenatal development is the process in which an embryo An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination. In humans, it is called an embryo until about eight weeks after fertilization (i.e. ten weeks LMP), and from then it is instead called a fetus or fetus A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth (or foetus) gestates Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time . The time interval of a gestation plus 2 weeks is called gestation period, and the length of time plus 2 weeks that the offspring have spent developing in the uterus is called gestational age during pregnancy Pregnancy is the carrying of one or more offspring, known as a fetus or embryo, inside the womb of a female. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets. Human pregnancy is the most studied of all mammalian pregnancies. Childbirth usually occurs about 38 weeks after conception; i.e., approximately 40 weeks, from fertilization Human fertilization is the union of a human egg and sperm, usually occurring in the ampulla of the uterine tube. It is also the initiation of prenatal development. Scientists discovered the dynamics of human fertilization in the nineteenth century until birth Birth is the act or process of bearing or bringing forth offspring. The offspring is brought forth from the mother. Different forms of birth are oviparity, vivipary and ovovivipary. Often, the terms fetal development, foetal development, or embryology Embryology is a science which is about the development of an embryo from the fertilization of the ovum to the fetus stage. After cleavage, the dividing cells, or morula, becomes a hollow ball, or blastula, which develops a hole or pore at one end are used in a similar sense.

After fertilization the embryogenesis Embryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus. It starts with the fertilization of the ovum by sperm. The fertilized ovum is referred to as a zygote. The zygote undergoes rapid mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading starts. In humans, when embryogenesis finishes, by the end of the 10th week of gestational age Gestational age is the age of an embryo or fetus . In humans, a common method of calculating gestational age starts counting either from the first day of the woman's last menstrual period (LMP) or from 14 days before conception (fertilization). Counting from the first day of the LMP involves the assumption that conception occurred 14 days later, the precursors of all the major organs of the body have been created. Therefore, the following period, the fetal period, is described both topically on one hand, i.e. by organ, and strictly chronologically on the other, by a list of major occurrences by weeks of gestational age.

Contents 1 Definitions of periods 2 Fertilization 3 Embryonic period 3.1 Changes by weeks of gestation 3.1.1 Gestational age vs. embryonic age 3.1.2 Week 1-2 3.1.3 Week 3 3.1.4 Week 4 3.1.5 Week 5 3.1.6 Week 6 3.1.7 Week 7 3.1.8 Week 8 3.1.9 Week 9 4 Fetal period 4.1 Changes by organ 4.2 Changes by weeks of gestation 4.2.1 Weeks 10-12 4.2.2 Weeks 13 to 16 4.2.3 Week 19 4.2.4 Week 23 4.2.5 Week 27 4.2.6 Week 31 4.2.7 Week 35 4.2.8 Weeks 36 to 39 5 See also 6 References 7 External links

Definitions of periods

Stages during pregnancy Pregnancy is the carrying of one or more offspring, known as a fetus or embryo, inside the womb of a female. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets. Human pregnancy is the most studied of all mammalian pregnancies. Childbirth usually occurs about 38 weeks after conception; i.e., approximately 40 weeks. Embryogenesis is marked in green. Weeks and months are numbered by gestation Gestational age is the age of an embryo or fetus . In humans, a common method of calculating gestational age starts counting either from the first day of the woman's last menstrual period (LMP) or from 14 days before conception (fertilization). Counting from the first day of the LMP involves the assumption that conception occurred 14 days later.

• The perinatal period (from Greek peri, "about, around" and Latin nasci "to be born") is "around the time of birth Childbirth is the culmination of a human pregnancy or gestation period with the birth of one or more newborn infants from a woman's uterus. The process of normal human childbirth is categorized in three stages of labour: the shortening and dilation of the cervix, descent and birth of the infant, and birth of the placenta. In many cases, with", specifically from 22 completed weeks (154 days) of gestation Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time . The time interval of a gestation plus 2 weeks is called gestation period, and the length of time plus 2 weeks that the offspring have spent developing in the uterus is called gestational age (the time when birth weight Birth mass is the mass of a baby at its birth. It has direct links with the gestational age at which the child was born and can be estimated during the pregnancy by measuring fundal height. A baby born within the normal range of mass for that gestational age is known as appropriate for gestational age . Those born above or below that range have is normally 500 g) to 7 completed days after birth.^[1]

• The antepartum period (from Latin ante "before" and parere "to give birth") is literally equivalent to prenatal (from Latin pre- "before" and nasci "to be born"). Practically, however, antepartum usually refers to the period between the 24th/26th week of gestational Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time . The time interval of a gestation plus 2 weeks is called gestation period, and the length of time plus 2 weeks that the offspring have spent developing in the uterus is called gestational age age until birth, for example in antepartum hemorrhage^[2][3].

Fertilization

Main article: Human fertilization Human fertilisation is the union of a human egg and sperm, usually occurring in the ampulla of the uterine tube. It is also the initiation of prenatal development. Scientists discovered the dynamics of human fertilisation in the nineteenth century A sperm fertilizing an ovum

When semen is deposited in the vagina, the spermatozoa travel through the cervix The cervix is the lower, narrow portion of the uterus where it joins with the top end of the vagina. It is cylindrical or conical in shape and protrudes through the upper anterior vaginal wall. Approximately half its length is visible with appropriate medical equipment; the remainder lies above the vagina beyond view. It is occasionally called & and body of the uterus and into the Fallopian tubes The Fallopian tubes, named after Gabriel Fallopius , also known as oviducts, uterine tubes, and salpinges (singular salpinx) are two very fine tubes lined with ciliated epithelia, leading from the ovaries of female mammals into the uterus, via the utero-tubal junction. In non-mammalian vertebrates, the equivalent structures are the oviducts. Fertilization Fertilisation , is the fusion of gametes to produce a new organism. In animals, the process involves the fusion of an ovum with a sperm, which eventually leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external of the ovum An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization. In lower plants and algae, the ovum is also often called oosphere (egg cell The cell is the functional basic unit of life. It was discovered by Robert Hooke and is the functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. Some organisms, such as most bacteria, are unicellular . Other organisms, such as humans,) usually takes place in the Fallopian tube. Many sperm must cooperate to penetrate the thick protective shell-like barrier that surrounds the ovum. The first sperm that penetrates fully into the egg donates its genetic material In modern molecular biology, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA (DNA Deoxyribonucleic acid ( /diːˌɒksɨˌraɪbɵ.nuːˈkleɪ.ɪk ˈæsɪd/ (help·info)) (DNA) is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information. DNA is often compared to a set of). The egg then polarizes, repelling any additional sperm. The resulting combination is called a zygote A zygote , or zygocyte, is the initial cell formed when a new organism is produced by means of sexual reproduction. A zygote is synthesized from the union of two gametes, and constitutes the first stage in a unique organism's development. Zygotes are usually produced by a fertilization event between two haploid cells — an ovum from a female and. The term "conception" refers variably to either fertilization or to formation of the conceptus Conceptus denotes the embryo and its adnexa (appendages or adjunct parts) or associated membranes (i.e. the products of conception) The conceptus includes all structures that develop from the zygote, both embryonic and extraembryonic. It includes the embryo as well as the embryonic part of the placenta and its associated membranes - amnion, after uterine implantation, and this terminology is controversial Controversy over the beginning of pregnancy usually occurs in the context of the abortion debate. Depending on where pregnancy is considered to begin, some methods of birth control or infertility treatment might be considered abortifacient. The controversy is not primarily a scientific issue since knowledge of human reproduction and development.

Prior to fertilization, each ovum contains a complete human genome In modern molecular biology, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA, including a single X but no Y chromosome The Y chromosome is one of the two sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 60 million base pairs. DNA in the Y chromosome is passed from father to son, thus tracking many surnames. Y-DNA analysis is. Likewise, each spermatozoon contains a complete set of autosomes An autosome is a chromosome that is not a sex chromosome – that is to say there are an equal number of copies of the chromosome in males and females. For example, in humans, there are 22 pairs of autosomes, and in addition there are X and Y chromosomes which are sex chromosomes and a single sex chromosome, either X or Y. The resulting zygote is similar to the majority of somatic cells because it contains two copies of the genome in a diploid Human germ cells have one complete set of chromosomes from the male or female parent. Germ cells, also called gametes, combine to produce somatic cells. Somatic cells therefore have twice as many chromosomes. The haploid number (n) is the number of chromosomes in a gamete. A somatic cell has twice that many chromosomes (2n) set of chromosomes A chromosome is an organized structure of DNA and protein that is found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions. The word chromosome comes from the Greek χρῶμα. One set of chromosomes came from the nucleus of the ovum and the second set from the nucleus of the sperm. If the spermatozoon contributes a Y chromosome then the zygote will develop as a male. Unlike the X chromosome The X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals . It is a part of the XY sex-determination system and X0 sex-determination system. The X chromosome was named for its unique properties by early researchers, which resulted in the naming of its counterpart Y chromosome, for the next letter in, the Y chromosome contains very little genetic information. However it does contain a gene, SRY SRY is a sex-determining gene on the Y chromosome in the therians (placental mammals and marsupials), which will switch on androgen production at a later stage, leading to the development of a male Male refers to the sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization. A male cannot reproduce sexually without access to at least one ovum from a female, but some organisms can reproduce both sexually body type. In contrast, the mitochondrial In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 10 micrometers (μm) in diameter. Mitochondria are sometimes described as "cellular power plants" because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of genetic information of the zygote comes entirely from the mother via the ovum.

Embryonic period

Main article: Human embryogenesis Human embryology is the study of human development during the first eight weeks from gametogenesis pre-conception through fertilization (conception of sperm and oocyte in the ampulla arm where the fallopian tube and ovary adjoin) up to and including the 8th week after implantation of the zygote (fertilized egg) in the uterus. After the 8th week of The initial stages of human embryogenesis Human embryology is the study of human development during the first eight weeks from gametogenesis pre-conception through fertilization (conception of sperm and oocyte in the ampulla arm where the fallopian tube and ovary adjoin) up to and including the 8th week after implantation of the zygote (fertilized egg) in the uterus. After the 8th week of

The embryonic period in humans begins at fertilization Fertilisation , is the fusion of gametes to produce a new organism. In animals, the process involves the fusion of an ovum with a sperm, which eventually leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external (12-24hrs after ovulation, generally between the 2nd and 3rd week of gestational age) and continues until the end of the 10th week of gestation (8th week by embryonic age).

The zygote spends the next few days traveling down the Fallopian tube The Fallopian tubes, named after Gabriel Fallopius , also known as oviducts, uterine tubes, and salpinges (singular salpinx) are two very fine tubes lined with ciliated epithelia, leading from the ovaries of female mammals into the uterus, via the utero-tubal junction. In non-mammalian vertebrates, the equivalent structures are the oviducts. Meanwhile it divides several times to form a ball of cells called a morula A morula is an embryo at an early stage of embryonic development, consisting of cells (called blastomeres) in a solid ball contained within the zona pellucida. Further cellular division Cell division is a process by which a cell, called the parent cell, divides into two or more cells, called daughter cells. Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort of cell division in is accompanied by the formation of a small cavity between the cells. This stage is called a blastocyst The blastocyst is a structure formed in the early embryogenesis of mammals, after the formation of the morula. It possesses an inner cell mass , or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast, which later forms the placenta. The trophoblast surrounds the inner cell mass and a fluid-filled. Up to this point there is no growth in the overall size of the embryo An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination. In humans, it is called an embryo until about eight weeks after fertilization (i.e. ten weeks LMP), and from then it is instead called a fetus, so each division produces successively smaller cells.

The blastocyst The blastocyst is a structure formed in the early embryogenesis of mammals, after the formation of the morula. It possesses an inner cell mass , or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast, which later forms the placenta. The trophoblast surrounds the inner cell mass and a fluid-filled reaches the uterus The uterus (from Latin "uterus" , plural uteruses or uteri) or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species. It is within the uterus that the fetus at roughly the fifth day after fertilization. It is here that lysis of the zona pellucida The zona pellucida is a glycoprotein membrane surrounding the plasma membrane of an oocyte. It is a vital constitutive part of the latter, external but not extraneous to it. The zona pellucida first appears in multilaminar primary oocytes, a glycoprotein shell, occurs. This is required so that the trophectoderm cells, which give rise to extra-embryonic structures such as the placenta The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. Placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and lizards with varying levels of development up to, of the blastocyst can come into contact with the luminal epithelial cells of the endometrium. (Contrast this with zona hatching Zona hatching is a phenomenon occurring during prenatal development. Prior to this event, the predecessor of the embryo, in the form of a blastocyst, is surrounded by a glycoprotein sphere called the zona pellucida. To be able to perform implantation on the uterine wall, the blastocyst first needs to get rid of the zona pellucida. This lysis of, an event that occurs in vitro A procedure performed in vitro is performed not in a living organism but in a controlled environment, such as in a test tube or Petri dish. Many experiments in cellular biology are conducted outside of organisms or cells; because the test conditions may not correspond to the conditions inside of the organism, this may lead to results that do not by a different mechanism, but with a similar result). It then adheres to the uterine lining and becomes embedded in the endometrial The endometrium functions as a lining for the uterus, preventing adhesions between the opposed walls of the myometrium, thereby maintaining the patency of the uterine cavity. During the menstrual cycle or estrous cycle, the endometrium grows to a thick, blood vessel-rich, glandular tissue layer. This represents an optimal environment for the cell layer. This process is also called implantation. In most successful pregnancies, the conceptus implants 8 to 10 days after ovulation (Wilcox et al. 1999). The inner cell mass forms the embryo, while the outer cell layers form the membranes and placenta. Together, the embryo and its membranes are referred to as a conceptus, or the "products of conception".

Rapid growth occurs and the embryo's main external features begin to take form. This process is called differentiation, which produces the varied cell types (such as blood cells, kidney cells, and nerve cells). A spontaneous abortion, or miscarriage, in the first trimester of pregnancy is usually due to major genetic mistakes or abnormalities in the developing embryo. During this critical period (most of the first trimester), the developing embryo is also susceptible to toxic exposures, such as:

• Alcohol, certain drugs, and other toxins that cause birth defects, such as Fetal alcohol syndrome
• Infection (such as rubella or cytomegalovirus)
• Radiation from x-rays or radiation therapy
• Nutritional deficiencies such as lack of folate which contributes to spina bifida

Generally, if a structure pre-dates another structure in evolutionary terms, then it often appears earlier than the other in an embryo; this general observation is sometimes summarized by the phrase "ontogeny recapitulates phylogeny."^[4] For example, the backbone is a common structure among all vertebrates such as fish, reptiles and mammals, and the backbone also appears as one of the earliest structures laid out in all vertebrate embryos. The cerebrum in humans, which is the most sophisticated part of the brain, develops last. The concept of recapitulation is not absolute, but it is recognized as being partly applicable to development of the human embryo.^[4]

Changes by weeks of gestation

See also: Embryo and Human embryogenesis

Gestational age vs. embryonic age

Gestational age is the time that has passed since the onset of the last menstruation, which generally or as standard occurs 2 weeks before the actual fertilization. Embryonic age, in contrast measures the actual age of the embryo or fetus from the time of fertilization. Nevertheless, menstruation has historically been the only means of estimating embryonal/fetal age, and is still the presumed measure if not else specified. However, the actual duration between last menstruation and fertilization may in fact differ from the standard 2 weeks by several days.

Thus, the first week of embryonic age is already week three counting with gestational age.

Furthermore, the number of the week is one more than the actual age of the embryo/fetus. For example, the embryo is 0 whole weeks old during the 1st week after fertilization.

The following table summarizes the various expression systems during week number x of gestation.

	Week number	Reached age (whole weeks)
Gestational	x	x-1
Embryonic	x-2	x-3

Week 1-2

Gestational age: 0 to 1 (whole) weeks old. 1–14 days from last menstruation.

Embryonic age: -2 to -1 weeks old. That is, week 1-2 of gestational age are merely theoretical extrapolations of embryonic age, since the fertilization hasn't actually occurred yet.

Week 3

Gestational age: 2 (whole) weeks old. 15–21 days from last menstruation.

Embryonic age: Week nr 1. 0 (whole) weeks old. 1–7 days from fertilization.

• Fertilization of the ovum to form a zygote. (day 1 of fert.^[5])
• The zygote undergoes mitotic cellular divisions, but does not increase in size. This mitosis is also known as cleavage. A hollow cavity forms marking the blastocyst stage. (day 1.5-3 of fert.^[5])
• The blastocyst contains only a thin rim of trophoblast cells and a clump of cells at one end known as the "embryonic pole" which include embryonic stem cells.
• The embryo hatches from its protein shell (zona pellucida) and performs implantation onto the endometrial lining of the mother's uterus. (day 5-6 of fert.^[5])
• If separation into identical twins occurs, 1/3 of the time it will happen before day 5.^[6]

Week 4

Gestational age: 3 weeks old. 22–28 days from last menstruation.

Embryonic age: Week nr 2. 1 week old. 8–14 days from fertilization.

• Trophoblast cells surrounding the embryonic cells proliferate and invade deeper into the uterine lining. They will eventually form the placenta and embryonic membranes. The blastocyst is fully implanted day 7-12 of fert.^[5]
• Formation of the yolk sac.
• The embryonic cells flatten into a disk, two cells thick.
• If separation into identical twins occurs, 2/3 of the time it will happen between days 5 and 9. If it happens after day 9, there is a significant risk of the twins being conjoined.
• Primitive streak develops. (day 13 of fert.^[5])
• Primary stem villi appear. (day 13 of fert.^[5])

Week 5

Gestational age: 4 weeks old. 29–35 days from last menstruation.

Embryonic age: Week nr 3. 2 weeks old. 15–21 days from fertilization.

• A notochord forms in the center of the embryonic disk. (day 16 of fert.^[5])
• Gastrulation commences. (day 16of fert.^[5])
• A neural groove (future spinal cord) forms over the notochord with a brain bulge at one end. Neuromeres appear. (day 18 of fert.^[5])
• Somites, the divisions of the future vertebra, form. (day 20 of fert.^[5])
• Primitive heart tube is forming. Vasculature begins to develop in embryonic disc. (day 20 of fert.^[5])

Embryo at 4 weeks after fertilization. The crown-rump length is about 0.2 inches.^[7] A 10mm embryo from an ectopic pregnancy, still in the oviduct. This embryo is about five weeks old (or from the seventh week of menstrual age).

Week 6

Gestational age: 5 weeks old. 36-42 days from last menstruation.

Embryonic age: Week nr 4. 3 weeks old. 22-28 days from fertilization.

• The embryo measures 4 mm (1/8 inch) in length and begins to curve into a C shape.
• The heart bulges, further develops, and begins to beat in a regular rhythm. Septum primum appear. ^[5]
• Branchial arches, grooves which will form structures of the face and neck, form.
• The neural tube closes.
• The ears begin to form as otic pits.
• Arm buds and a tail are visible.
• Pulmonary primordium, the first traits of the lung appear. ^[5]
• Hepatic plate, the first traits of the liver appear. ^[5]
• Buccopharyngeal membrane ruptures. This is the future mouth. ^[5]
• Cystic diverticulum, which will become the gallbladder, and dorsal pancreatic bud, which will become the pancreas appear. ^[5]
• Urorectal septum begins to form. Thus, the rectal and urinary passageways become separated. ^[5]
• Anterior and posterior horns differentiate in the spinal cord ^[5]
• Spleen appears. ^[5]
• Ureteric buds appear. ^[5]

This embryo is also from an ectopic pregnancy, this one in the cornu (the part of the uterus to which the Fallopian tube is attached). The features are consistent with a developmental age of seven weeks (reckoned as the ninth week of pregnancy).

Week 7

Gestational age: 6 weeks old. 43–49 days from last menstruation.

Embryonic age: Week nr 5. 4 weeks old. 29–35 days from fertilization.

• The embryo measures 8 mm (1/4 inch) in length.
• Lens pits and optic cups form the start of the developing eye.
• Nasal pits form.
• The brain divides into 5 vesicles, including the early telencephalon.
• Leg buds form and hands form as flat paddles on the arms.
• Rudimentary blood moves through primitive vessels connecting to the yolk sac and chorionic membranes.

Week 8

Gestational age: 7 weeks old. 50–56 days from last menstruation.

Embryonic age: Week nr 6. 5 weeks old. 36–42 days from fertilization.

• The embryo measures 13 mm (1/2 inch) in length.
• Lungs begin to form.
• The brain continues to develop.
• Arms and legs have lengthened with foot and hand areas distinguishable.
• The hands and feet have digits, but may still be webbed.
• The gonadal ridge begins to be perceptible.
• The lymphatic system begins to develop.

Week 9

A six week embryonic age or eight week gestational age intact Embyo

Gestational age: 8 weeks old. 57–63 days from last menstruation.

Embryonic age: Week nr 7. 6 weeks old. 43–49 days from fertilization.

• The embryo measures 18 mm (3/4 inch) in length.
• Fetal heart tone (the sound of the heart beat)can be heard using doppler.
• Nipples and hair follicles begin to form.
• Location of the elbows and toes are visible.
• Spontaneous limb movements may be detected by ultrasound.
• All essential organs have at least begun

Fetal period

The fetal period begins at the end of the 10th week of gestation (8th week of development). Since the precursors of all the major organs are created by this time, the fetal period is described both by organ and by a list of changes by weeks of gestational age.

Because the precursors of the organs are formed, fetus also is not as sensitive to damage from environmental exposures as the embryo. Instead, toxic exposures often cause physiological abnormalities or minor congenital malformation.

Changes by organ

Each organ has its own development.

• Development of circulatory system
  • Heart development
• Development of digestive system
  • Tooth development
• Development of endocrine system
• Development of integumentary system
• Development of lymphatic system
• Development of muscular system
• Development of nervous system
• Development of the urinary and reproductive system
  • Development of the reproductive system
    • Development of the gonads
• Development of respiratory system

Changes by weeks of gestation

From the 8th week until birth (around 38 weeks), the developing organism is called a fetus. The fetus is not as sensitive to damage from environmental exposures as the embryo, and toxic exposures often cause physiological abnormalities or minor congenital malformation. All major structures are already formed in the fetus, but they continue to grow and develop.

Fetus at 8 weeks after fertilization.^[8]

Weeks 10-12

Gestational age: 9–11 weeks old.

Embryonic age: Weeks nr 8-10. 7–9 weeks old.

• Embryo measures 30 mm-8 cm (1.2-3.2 inches) in length.
• Intestines rotate.
• Facial features continue to develop.
• The eyelids are more developed.
• The external features of the ear begin to take their final shape.
• The head comprises nearly half of the fetus' size.
• The face is well formed
• The eyelids close and will not reopen until about the 28th week.
• Tooth buds, which will form the baby teeth, appear.
• The limbs are long and thin.
• The fetus can make a fist with its fingers.
• Genitals appear well differentiated.
• Red blood cells are produced in the liver.

Weeks 13 to 16

Gestational age: 12–15 weeks old.

Embryonic age: Weeks nr 11-14. 10–13 weeks old.

• The fetus reaches a length of about 15 cm (6 inches).
• A fine hair called lanugo develops on the head.
• Fetal skin is almost transparent.
• More muscle tissue and bones have developed, and the bones become harder.
• The fetus makes active movements.
• Sucking motions are made with the mouth.
• Meconium is made in the intestinal tract.
• The liver and pancreas produce fluid secretions.
• From week 13, sex prediction by obstetric ultrasonography is almost 100% accurate.^[9]

Fetus at 18 weeks after fertilization.^[10]

Week 19

Gestational age: 18 weeks old.

Embryonic age: Week nr 17. 16 weeks old.

• The fetus reaches a length of 20 cm (8 inches).
• Lanugo covers the entire body.
• Eyebrows and eyelashes appear.
• Nails appear on fingers and toes.
• The fetus is more active with increased muscle development.
• "Quickening" usually occurs (the mother and others can feel the fetus moving).
• The fetal heartbeat can be heard with a stethoscope.

Week 23

Gestational age: 22 weeks old.

Embryonic age: Week nr 21. 20 weeks old.

• The fetus reaches a length of 28 cm (11.2 inches).
• The fetus weighs about 725 g (1 lb 10 oz).
• Eyebrows and eyelashes are well formed.
• All of the eye components are developed.
• The fetus has a hand and startle reflex.
• Footprints and fingerprints continue forming.
• Alveoli (air sacs) are forming in lungs.

Week 27

Gestational age: 26 weeks old.

Embryonic age: Week nr 25. 24 weeks old.

• The fetus reaches a length of 38 cm (15 inches).
• The fetus weighs about 1.2 kg (2 lb 11 oz).
• The brain develops rapidly.
• The nervous system develops enough to control some body functions.
• The eyelids open and close.
• The cochleae are now developed, though the myelin sheaths in neural portion of the auditory system will continue to develop until 18 months after birth.
• The respiratory system, while immature, has developed to the point where gas exchange is possible.

Week 31

Gestational age: 30 weeks old.

Embryonic age: Week nr 29. 28 weeks old.

• The fetus reaches a length of about 38–43 cm (15-17 inches).
• The fetus weighs about 2 kg (3 lb 0 oz).
• The amount of body fat rapidly increases.
• Rhythmic breathing movements occur, but lungs are not fully mature.
• Thalamic brain connections, which mediate sensory input, form.
• Bones are fully developed, but are still soft and pliable.
• The fetus begins storing iron, calcium, and phosphorus.

Week 35

Gestational age: 34 weeks old.

Embryonic age: Week nr 33. 32 weeks old.

• The fetus reaches a length of about 40–48 cm (16-19 inches).
• The fetus weighs about 2.5 to 3 kg (5 lb 12 oz to 6 lb 12 oz).
• Lanugo begins to disappear.
• Body fat increases.
• Fingernails reach the end of the fingertips.
• A baby born at 36 weeks has a high chance of survival, but may require medical interventions.

Fetus at 38 weeks after fertilization.^[11]

Weeks 36 to 39

Gestational age: 35–38 weeks old.

Embryonic age: Weeks nr 34-37. 33–36 weeks old.

• The fetus is considered full-term at the end of the 37th week of gestational age.
• It may be 48 to 53 cm (19 to 21 inches) in length.
• The lanugo is gone except on the upper arms and shoulders.
• Fingernails extend beyond fingertips.
• Small breast buds are present on both sexes.
• Head hair is now coarse and thickest.

The development is continued postnatally with adaptation to extrauterine life and child development stages.

See also

• Environmental toxins and fetal development

References

1. ^ Definitions and Indicators in Family Planning. Maternal & Child Health and Reproductive Health. By European Regional Office, World Health Organization. Revised March 1999 & January 2001. In turn citing: WHO Geneva, WHA20.19, WHA43.27, Article 23
2. ^ patient.co.uk » PatientPlus » Antepartum Haemorrhage Last Updated: 5 May 2009
3. ^ The Royal Women’s Hospital > antepartum haemorrhage Retrieved on Jan 13, 2009
4. ^ ^{a b} Stephen Jay Gould,. Ontogeny and Phylogeny. Cambridge, Mass: Belknap Press. p. 206. ISBN 0-674-63941-3. http://www.sjgarchive.org/library/ontogeny.html.
5. ^ ^{a b c d e f g h i j k l m n o p q r s t} )William J. Larsen (2001). Human embryology. Edinburgh: Churchill Livingstone. ISBN 0-443-06583-7.
6. ^ Scott F. Gilbert; with a chapter on plant development by Susan R. Singer (2000). Developmental biology. Sunderland, Mass: Sinauer Associates. ISBN 0-87893-243-7. http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=dbio.box.2669.
7. ^ 3D Pregnancy (large image of fetus at 4 weeks after fertilization). Retrieved 2007-08-28. A rotatable 3D version of this photo is available here, and a sketch is available here.
8. ^ 3D Pregnancy (large image of fetus at 10 weeks after fertilization). Retrieved 2007-08-28. A rotatable 3D version of this photo is available here, and a sketch is available here.
9. ^ Mazza V, Falcinelli C, Paganelli S, et al. (June 2001). "Sonographic early fetal gender assignment: a longitudinal study in pregnancies after in vitro fertilization". Ultrasound Obstet Gynecol 17 (6): 513–6. doi:10.1046/j.1469-0705.2001.00421.x. PMID 11422974.
10. ^ 3D Pregnancy (large image of fetus at 18 weeks after fertilization). Retrieved 2007-08-28. A rotatable 3D version of this photo is available here, and a sketch is available here.
11. ^ 3D Pregnancy (large image of fetus at 38 weeks after fertilization). Retrieved 2007-08-28. A rotatable 3D version of this photo is available here, and a sketch is available here.

• "MedlinePlus Medical Encyclopedia"
• Moore, Keith L. The Developing Human: 3rd Edition. W.B. Saunders Company, Philadelphia PA
• Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy. 1999 N Engl J Med. 340(23):1796-9. PMID 10362823
• Ljunger, E, Cnattingius, S, Lundin, C, & Annerén, G. 2005 Chromosomal anomalies in first-trimester miscarriages. Acta Obstetricia et Gynecologica Scandinavica 84(11):1103-1107. PMID 10362823

External links

Wikimedia Commons has media related to: Embryology

• Fetal Development Timeline from AboutKidsHealth.ca
• The Changes in Each Stage of Human Development
• Real Time Presentation of Human Embryo Development
• Fetal development week by week at www.babycentre.co.uk

Human Development: Biological • Psychological Pre- and perinatal Biological Prenatal development Psychological Pre- and perinatal psychology Infancy Biological Infant Psychological Infant and child psychology Childhood Biological Child development · Child development stages · Toddler Psychological Infant and child psychology · Preadolescence Adolescence Biological Puberty Psychological Youth development · Adolescent psychology Young adulthood Psychological Young adult (psychology) Middle adulthood Biological Middle age Maturity Biological Ageing · Senescence Psychological Old age Legal and general definitions Minor (law) · Infancy · Child · Childhood · Adolescence · Age of majority · Adult Theorists-theories John Bowlby-attachment · Urie Bronfenbrenner-ecological systems · Erik Erikson-psychosocial · Sigmund Freud-psychosexual · Lawrence Kohlberg-moral · Jean Piaget-cognitive · Lev Vygotsky-cultural-historical

Human physiology and Endocrinology of Sexual reproduction Menstrual/Estrous cycle Menstruation · Follicular phase · Ovulation · Luteal phase Gametogenesis Spermatogenesis (spermatogonium, spermatocyte, spermatid, sperm) · Oogenesis (oogonium, oocyte, ootid, ovum) · Germ cell (gonocyte, gamete) Human sexual behavior Sexual intercourse · Masturbation · Erection · Orgasm · Ejaculation · Insemination · Fertilisation/Fertility · Implantation · Pregnancy · Postpartum period Life span Prenatal development/Sexual dimorphism/Sexual differentiation (Feminization, Virilization) · Puberty (Gonadarche, Pubarche, Menarche, Adrenarche) · Maternal age / Paternal age · Climacteric (Menopause, Andropause) Egg (biology) Ovum · Oviposition · Oviparity · Ovoviviparity · Vivipary Reproductive endocrinology and infertility Hypothalamic-pituitary-gonadal axis · Andrology · Hormone Breast Thelarche · Lactation · Breastfeeding : ♀ FRS // //, / /, drug (//) : ♂ MRS // //, / , drug (/) : BRE / //

Pregnancy and Childbirth Planning Birth control · Pre-conception counseling Conception Fertility awareness · Assisted reproductive technology (IVF, AI, Fertility medication) Testing Home testing · Obstetric ultrasonography · Prenatal diagnosis · 3D ultrasound Prenatal Anatomy Endometrium · Placenta · Amniotic fluid · Amniotic sac Development Gestational age · Fundal height · Human embryogenesis · Maternal physiological changes Care Nutrition Procedures Amniocentesis · Chorionic villus sampling Childbirth Preparation Lamaze · Bradley method · Nesting instinct · Adaptation to extrauterine life Roles Midwife · Doula · Obstetrician · Men's roles Delivery Pelvimetry/Bishop score: Cervical effacement · Cervical dilation · Position Unassisted childbirth · Home birth · Water birth · Natural childbirth Rupture of membranes · Bloody show · Contraction · Childbirth positions · Presentation (Cephalic presentation, Breech presentation, Shoulder presentation) Postpartum Childcare · Congenital disorders · Sex after pregnancy G/P Gravida/para/abortus · TPAL : OBS / //, , drug(/)

Developmental biology > Human embryogenesis (development of embryo) and development of fetus First three weeks Week 1 Fertilization · Egg activation · Zygote · Cleavage · Morula · Blastula (Blastomere) · Blastocyst · Inner cell mass Week 2 (Bilaminar) Hypoblast · Epiblast Week 3 (Trilaminar) Germ layers Archenteron/Primitive streak (Primitive pit, Primitive knot/Blastopore, Primitive groove) · Gastrula/Gastrulation · Regional specification Ectoderm Surface ectoderm · Neuroectoderm · Somatopleure · Neurulation · Neural crest Endoderm Splanchnopleure Mesoderm Chorda- · Paraxial (Somite/Somitomere/Sclerotome/Myotome/Dermatome) · Intermediate · Lateral plate (Intraembryonic coelom, Splanchnopleure/Somatopleure) Extraembryonic/ uterus Trophoblast (Cytotrophoblast, Syncytiotrophoblast) Blastocoele · Yolk sack/exocoelomic cavity · Heuser's membrane · Extraembryonic coelom · Vitelline duct Umbilical cord (Umbilical artery, Umbilical vein, Wharton's jelly) · Allantois Placenta · Decidua (Decidual cells) · Chorionic villi/Intervillous space · Gestational sac (Amnion/Amniotic sac/Amniotic cavity, Chorion) Histogenesis Programmed cell death (Apoptosis) · Stem cells · Germ line development Organogenesis Limb development: Limb bud · Apical ectodermal ridge/AER other structures: Eye development · Cutaneous structure development · Heart development · Development of the urinary and reproductive organs (Some dates are approximate—see Carnegie stages and a timeline.) : OBS / //, , drug(/)

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