Fetal development

Prenatal development is the process in which an embryo or fetus (or foetus) gestates during pregnancy, from the times of fertilization until birth. Often, the terms fetal development, foetal development, or embryology are used in a similar sense.

Fertilization and embryogenesis
When semen is deposited in the vagina, the spermatozoa travel through the cervix and body of the uterus and into the Fallopian tubes. Fertilization of the ovum (egg cell) 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 (DNA). The resulting combination is called a zygote. The term "conception" refers variably to either fertilization or to formation of the conceptus after uterine implantation, and this terminology is controversial.

Like every cell in the body, the zygote contains all of the genetic information unique to an individual. Half of the genetic information residing in the zygote's nucleus comes from the mother's egg nucleus, and the other half from the nucleus of a single sperm. However, the mitochondrial genetic information of the zygote is in its totality contributed by the mother's egg. The zygote spends the next few days traveling down the Fallopian tube. Meanwhile it divides several times to form a ball of cells called a morula. Further cellular division is accompanied by the formation of a small cavity between the cells. This stage is called a blastocyst. Up to this point there is no growth in the overall size of the embryo, so each division produces successively smaller cells.

The blastocyst reaches the uterus at roughly the fifth day after fertilization. It is here that lysis of the zona pellucida, a glycoprotein shell, occurs. This is required so that the trophectoderm cells of the blastocyst can come into contact with the luminal epithelial cells of the endometrium. (Contrast this with zona "hatching", an event that occurs in vitro by a different mechanism, but with a similar result). It then adheres to the uterine lining and becomes embedded in the endometrial 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 Spectrum Disorder
 * 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 also appears earlier than the other in the embryo. This general rule is sometimes summarized by the phrase "ontogeny recapitulates phylogeny." 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 earlier structures laid out in all vertebrate embryos. The cerebrum, which is the most sophisticated part of the brain, develops last. This rule is not absolute, but it is recognized as being partly applicable to development of the embryo.

Fetal development
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. The majority of structures are already formed in the fetus, but they continue to grow and become functional.

Changes by weeks of gestational age
The following list describes specific changes in development by week of gestational age. This is the age from the beginning of the last menstrual period, and is often calculated by using a combination of the last menstrual period, physical examination and ultrasound of the patient, and, if applicable, history of assisted reproduction. By convention, the gestational age is two weeks more than the developmental or conceptional age.

Embryonic period
Toxic exposures during the first two weeks following fertilization (second and third weeks of gestational age) may cause prenatal death but do not cause developmental defects. Subsequent toxic exposures in the embryonic period often cause major congenital malformations.


 * Week 2 (week of fertilization)
 * Fertilization of the ovum to form a zygote which undergoes mitotic cellular division, but does not increase in size. A hollow cavity forms marking the blastocyst stage.
 * 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 blastocyst hatches from its protein shell (zona pellucida) and implants onto the endometrial lining of the mother's uterus.
 * If separation into identical twins occurs, 1/3 of the time it will happen before day 5.


 * Week 3 (1 week following fertilization)
 * Trophoblast cells surrounding the embryonic cells proliferate and invade deeper into the uterine lining. They will eventually form the placenta and embryonic membranes.
 * 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.


 * Week 4 (2 weeks from fertilization - first missed menstrual period)
 * A notochord forms in the center of the embryonic disk.
 * A neural groove (future spinal cord) forms over the notochord with a brain bulge at one end.
 * Heart tubes begin to fuse.


 * Week 5 (3 weeks from fertilization)
 * The embryo measures 4 mm (1/8 inch) in length and begins to curve into a C shape.
 * Somites, the divisions of the future vertebra, form.
 * The heart bulges, further develops, and begins to beat in a regular rhythm.
 * 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.


 * Week 6 (4th week of development)
 * The embryo measures 8 mm (1/4 inch) in length.
 * Lens pits and optic cups form the start of the developing eye.
 * A primitive mouth and 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 7 (5th week of development)
 * 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.


 * Week 8 (6th week of development)
 * The embryo measures 18 mm (3/4 inch) in length.
 * 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 formation.


 * Week 9 (7th week of development)
 * Embryo measures 30 mm (1.2 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.

Fetal period
During the fetal period, toxic exposures often cause physiological abnormalities or minor congenital malformation
 * Weeks 10 to 13 (8th to 11th week of development)
 * The fetus reaches a length of 8 cm (3.2 inches).
 * 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 14 to 17 (12th to 15th week of development)
 * 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.


 * Week 20 (18th week of development)
 * 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 can feel the fetus moving).
 * The fetal heartbeat can be heard with a stethoscope.


 * Week 24 (22nd week of development)
 * 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 28 (26th week of development)
 * 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.
 * A baby born prematurely at this time may survive, but the possibilities for complications and death remain high.


 * Weeks 33 (29th week of development)
 * The fetus reaches a length of about 38-43 cm (15-17 inches).
 * The fetus weighs about 2 kg (4 lb 6 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 36 (34th week of development)
 * 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.


 * Weeks 37 to 40 (35th to 38th week of development)
 * The fetus is considered full-term at 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 thicker

Non-human prenatal development
Most mammals develop similarly to homo sapiens; during the earliest stages of development, the embryo is largely indistinguishable from another mammal. The anatomy of the area surrounding an embryo or fetus, however, is different in litter-bearing animals compared to humans: each unborn animal is surrounded by placental tissue and is lodged along one of two long uteri instead of the single uterus found in a human female.