b. Fetal period
7.5 cm (3 in.),
30 g (1 oz)
18 cm (6.5-7 in.),
100 g (4 oz)
32-42 cm (13-17 in.)
1110-1350 g (2.5-3 lb)
Head is about half the length of the body, and length
Brain continues to enlarge.
Face is broad, with eyes fully developed, closed, and
External ears develop and are low set.
Bone formation continues.
Gender is distinguishable from external genitals.
Red bone marrow, thymus, and spleen participate in
blood cell formation.
Fetus begins to move, but movements cannot yet be felt
by the mother.
Heartbeat can be detected.
Body systems continue to develop.
Head is relatively smaller than rest of body.
Lower limbs lengthen.
Fetus appears even more humanlike.
Rapid development of body systems occurs.
17-20 w eeks
25-30 cm (10-12 in.),
200-450 g (0.5-1 lb)
Head is more proportionate to rest of body.
Eyebrows and head hair are visible.
Vernix caseosa (fatty secretions of oil glands
and dead epithelial cells) and lanugo (deli-
cate fetal hair) cover fetus.
Fetal movements are commonly felt by mother
Growth slows but lower limbs continue to
21-25 w eeks
27-35 cm (11-14 in.),
550-800 g (1.25-1.5 lb)
Head becomes even more proportionate to
rest of body.
Weight gain is substantial, and skin is pink and
Head and body are more proportionate
and eyes are open.
Toenails are visible.
Body fat is 3.5% of total body mass.
Testes begin to descend toward scrotum
Red bone marrow is major site of blood
41-45 cm (16.5-18 in.),
2000-2300 g (4.5-5 lb)
Skin is pink and smooth.
Fetus assum es upside-down position.
Body fat is
% of total body mass.
50 cm (20 in.),
3200-3400 g (7-7.5 lb)
Skin is usually bluish-pink, and growth
slows as birth approaches.
Body fat is 16% of total body mass.
Testes are usually in scrotum in full-term
Even after birth, an infant is not com-
pletely developed; an additional year is
required, especially for complete devel-
opment of the nervous system.
Pregnancy Lasts from Fertilization to Delivery