Reproductive System
Amniotic fluid
Umbilical cord
Uterine wall
Fetal Blood and Circulation
Throughout fetal development, the maternal blood supplies
oxygen and nutrients and carries away wastes. These sub-
stances diffuse between the maternal and fetal blood through
the placental membrane, and the umbilical blood vessels
carry them to and from the fetus
(f g. 23.24)
. Consequently,
the fetal blood and cardiovascular system are adapted to
intrauterine existence. For example, the concentration of
A fetus can receive medical treatments. Tubes can be inserted to
remove abnormal F
uid accumulations, such as in the bladder or in
the brain, and the problems corrected after birth. One young lady,
Macie Hope McCartney, had a tumor the size of a grapefruit at the
base of her spine removed 10 weeks before her birth. She was gen-
tly lifted from the uterus, remained attached during surgery, and was
then tucked back into the uterus to ±
nish development.
FIGURE 23.23
A full-term fetus is usually positioned with its head near the cervix.
Stages of Prenatal Development
Time Period
Major Events
Preembryonic stage
²irst week
Cells undergo mitosis, blastocyst forms; inner cell mass appears; blastocyst implants in uterine wall
Size: 1/4 inch (0.63 centimeter), weight: 1/120 ounce (0.21 gram)
Embryonic stage
Second through
eighth week
Inner cell mass becomes embryonic disc; primary germ layers form, embryo proper becomes cylindrical; main
internal organs and external body structures appear; placenta and umbilical cord form, embryo proper is
suspended in amniotic F
Size: 1 inch (2.5 centimeters), weight: 1/30 ounce (0.8 gram)
²etal stage
Ninth through
twelfth week
cation centers appear in bones, sex organs di³
erentiate, nerves and muscles coordinate so that the fetus can
move its limbs
Size: 4 inches (10 centimeters), weight: 1 ounce (28 grams)
Thirteenth through
sixteenth week
Body grows rapidly; ossi±
cation continues
Size: 8 inches (20 centimeters), weight: 6 ounces (170 grams)
Seventeenth through
twentieth week
Muscle movements are stronger, and woman may be aware of slight F
utterings; skin is covered with ±
ne downy
hair (lanugo) and coated with sebum mixed with dead epidermal cells (vernix caseosa)
Size: 12 inches (30.5 centimeters), weight: 1 pound (454 grams)
rst through
thirty-eighth week
Body gains weight, subcutaneous fat deposited; eyebrows and lashes appear; eyelids reopen; testes descend
Size: 21 inches (53 centimeters), weight: 6 to 10 pounds (2.7 to 4.5 kilograms)
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