895
CHAPTER TWENTY-THREE
Reproductive System
Amniotic fluid
Umbilical cord
Placenta
Uterine wall
Cervix
Amniochorionic
membrane
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.
TABLE
23.3
|
Stages of Prenatal Development
Stage
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
uid
Size: 1 inch (2.5 centimeters), weight: 1/30 ounce (0.8 gram)
²etal stage
Ninth through
twelfth week
Ossi±
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)
Twenty-±
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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