Week by Week
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.
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
- 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
- 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
- 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.
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
- Somites, the divisions of the future vertebra, form.
- The heart bulges, further develops, and begins to beat in a regular
- Branchial arches, grooves which will form structures of the face and neck,
- 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.
- Intestines rotate.
- Facial features continue to develop.
- the eyelids are more developed.
- the external features of the ear begin to take their final shape.
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
- 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
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