Essentials of Anatomy and Physiology

hCG is to stimulate the corpus luteum in the maternal
ovary so that it will continue to secrete estrogen and
progesterone. The secretion of progesterone is partic-
ularly important to prevent contractions of the
myometrium, which would otherwise result in miscar-
riage of the embryo. Once hCG enters maternal cir-
culation, it is excreted in urine, which is the basis for
many pregnancy tests. Tests for hCG in maternal
blood are even more precise and can determine
whether a pregnancy has occurred even before a men-
strual period is missed.
The corpus luteum is a small structure, however,
and cannot secrete sufficient amounts of estrogen and
progesterone to maintain a full-term pregnancy. The
placenta itself begins to secrete estrogenand proges-
teronewithin a few weeks, and the levels of these hor-
mones increase until birth. As the placenta takes over,

the secretion of hCG decreases, and the corpus

luteum becomes non-functional. During pregnancy,

estrogen and progesterone inhibit the anterior pitu-

itary secretion of FSH and LH, so no other ovarian

follicles develop. These placental hormones also pre-

pare the mammary glands for lactation.

The placenta also secretes relaxin, which helps

inhibit contractions of the myometrium. Relaxin also

permits stretching of the ligaments of the pubic sym-

physis so that this joint will give a little, if necessary,

during birth.

PARTURITION AND LABOR

Parturitionis the rather formal term for birth, and

laboris the sequence of events that occur during

birth. The average gestation period is 40 weeks (280

Human Development and Genetics 481

BOX21–2 FETAL DIAGNOSIS

certain genetic diseases is another reason a preg-

nant woman may wish to have this procedure.

Chorionic Villus Sampling (CVS)

In this procedure, a biopsy catheter is inserted

through the vagina and cervix to collect a small

portion of the chorionic villi. These cells are derived

from the fetus but are not part of the fetus itself.

The information obtained is the same as that for

amniocentesis, but CVS may be performed earlier in

pregnancy, at about 8 weeks. Although there is a

risk that the procedure may cause a miscarriage,

CVS is considered comparable in safety to amnio-

centesis. It is important to remember that no inva-

sive procedure is without risks.

Maternal Blood Tests

Alpha-fetoprotein (AFP) is produced by the fetus

and is found in maternal circulation. The level

reaches a peak between 12 and 15 weeks of gesta-

tion, and should then decrease. If AFP is still high

after 16 to 18 weeks, there is a 95% chance that the

fetus has spina bifida or anencephaly, malforma-

tions of the central nervous system. Recall from

Chapter 17 that sufficient folic acid during preg-

nancy will greatly reduce the risk to the fetus.

Maternal blood levels of pregnancy-associated

plasma protein A (PAPP-A) and beta hCG can be

measured during the first trimester. These tests, in

conjunction with ultrasound, can reliably detect

Down syndrome.

Several procedures are currently available to deter-

mine certain kinds of abnormalities in a fetus or to

monitor development.

Ultrasound (or Fetal Ultrasonography)

This is a non-invasive procedure; high-frequency

sound waves are transmitted through the abdomi-

nal wall into the uterus. The reflected sound waves

are converted into an image called a sonogram.

This method is used to confirm multiple pregnan-

cies, fetal age or position, or to detect fetal abnor-

malities such as heart defects or malformations

of other organs. Ultrasound may also be used to

determine the thickness of the fetal neck (nuchal

translucency), which is an indicator of Down syn-

drome.

Amniocentesis

This procedure is usually performed at 16 to 18

weeks of gestation. A hypodermic needle is inserted

through the wall of the abdomen into the amniotic

sac, and about 10 to 20 mL of amniotic fluid is

removed. Within this fluid are fetal cells, which can

be cultured so that their chromosomes may be

examined. Through such examination and bio-

chemical tests, a number of genetic diseases or

chromosome abnormalities may be detected.

Because women over the age of 35 years are

believed to have a greater chance of having a child

with Down syndrome, amniocentesis is often rec-

ommended for this age group. A family history of