b. Fetal period
Eye
Upper
limb
Lower
limb
Ear
Eye
Nose
Mouth
Upper
limb
Umbilical
cord
Lower
limb
Ear
Eye
Nose
Mouth
Upper
limb
Lower
limb
Ten-week fetus
Thirteen-week fetus
Twenty-six-week fetus
9-12
weeks
13-16 weeks
26-29
weeks
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
nearly doubles.
Brain continues to enlarge.
Face is broad, with eyes fully developed, closed, and
widely separated.
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
(quickening).
Growth slows but lower limbs continue to
lengthen.
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
wrinkled.
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
(28-32 weeks).
Red bone marrow is major site of blood
cell production.
30-34 weeks
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
8
% of total body mass.
35-38
w eeks
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
male infants.
Birth
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
495
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