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, which occurs after uterine implantation.
Like every cell in the body, the zygote contains all of the genetic information unique to an individual. Half of the genetic information came from the mother's egg, and the other half from a single sperm. 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. The embryo "hatches" from its zona pellucida, a glycoprotein shell. It then adheres to the uterine lining and becomes embedded in the endometrial cell layer. This process is also called "implantation". In most successful human 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 pregancy is usually due to major genetic mistakes or abnormalities in the developing embryo. During this critical period (most of the first trimester), the developing fetus is also susceptible to toxic exposures, such as:
- Alcohol, certain drugs, and other toxins that cause birth defects
- 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
From the 9th week until birth (around 38 weeks), the developing human 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 age (and weeks of pregnancy)
The following list describes specific changes in human development by week. "Weeks of pregnancy" are dated by obstetricians from the start of the last menstrual period which means that ovulation occurs at the end of the 2nd week.
Toxic exposures may cause prenatal death but do not cause developmental defects
Week 1 (3rd week of pregnancy)
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 the zygote is going to separate into identical twins, 1/3 of the time it will happen before day 5.
Week 2 (4th week of pregnancy)
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 the zygote is going to separate into identical twins, 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.
Toxic exposures often cause major congenital malformations
Week 3 (5th week of pregnancy - 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 4 (6th week of pregnancy)
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 5 (7th week of pregnancy)
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 6 (8th week of pregnancy)
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 7 (9th week of pregnancy)
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 8 (10th week of pregnancy)
Embryo measures 30 mm (1.2 inches) in length.
Facial features continue to develop.
the eyelids are more developed.
the external features of the ear begin to take their final shape.
During the fetal period, toxic exposures often cause physiological abnormalities or minor congenital malformation
Weeks 9 to 12 (11th to 14th week of pregnancy)
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 and develops a human appearance.
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 (15th to 18th week of pregnancy)
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 18 (20th week of pregnancy)
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 22 (24th week of pregnancy)
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.
Weeks 23 to 26 (25th to 28th week of pregnancy)
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 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 27 to 31 (29th to 33rd week of pregnancy)
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 34 (36th week of pregnancy)
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 35 to 38 (37th to 40th week of preganancy)
The fetus is considered full-term at the 37th week of pregnancy.
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.