634
Chapter 17
can result in decreased growth rate and death of the fetus.
The factors responsible for eclampsia are unknown, but the
evidence strongly implicates abnormal vasoconstriction of the
maternal blood vessels and inadequate invasion of the endo-
metrium by trophoblast cells, resulting in poor blood perfu-
sion of the placenta.
Pregnancy Sickness
Some women suffer from
pregnancy sickness
(popularly called
morning sickness), which is characterized by nausea and vom-
iting during the fi rst three months (fi rst trimester) of preg-
nancy. The exact cause is unknown, but high concentrations
of estrogen and other substances may be responsible.
Parturition
A normal human pregnancy lasts approximately 40 weeks,
counting from the fi rst day of the last menstrual cycle, or
approximately 38 weeks from the day of ovulation and con-
ception. Safe survival of premature infants is now possible at
about the twenty-fourth week of pregnancy, but treatment of
these infants often requires heroic efforts and often with sig-
nifi cant defi cits in the infant.
During the last few weeks of pregnancy, a variety of
events occur in the uterus, culminating in the birth (delivery)
of the infant, followed by the placenta. All of these events,
including delivery, are termed
parturition.
Throughout most
of pregnancy, the smooth muscle cells of the myometrium
are relatively disconnected from each other, and the uterus is
sealed at its outlet by the fi rm, infl exible collagen fi bers that
constitute the cervix. These features are maintained mainly
by progesterone. During the last few weeks of pregnancy, as a
result of ever-increasing levels of estrogen, the smooth muscle
cells synthesize
connexin,
proteins that form gap junctions
between the cells, which allows the myometrium to undergo
coordinated contractions. Simultaneously, the cervix becomes
soft and fl exible due to an enzymatically mediated breakup of
its collagen fi bers. The synthesis of the enzymes is mediated
by a variety of messengers, including estrogen and placental
prostaglandins, the synthesis of which is stimulated by estro-
gen. The peptide hormone
relaxin
secreted by the ovaries is
also involved. Estrogen has yet another important effect on
the myometrium during this period: It induces the synthesis of
receptors for the posterior pituitary hormone oxytocin, which
is a powerful stimulator of uterine smooth muscle contraction.
Delivery is produced by strong rhythmical contractions
of the myometrium. Actually, weak and infrequent uterine
contractions begin at approximately 30 weeks and gradu-
ally increase in both strength and frequency. During the last
month, the entire uterine contents shift downward so that
the near-term fetus is brought into contact with the cervix.
In over 90 percent of births, the baby’s head is downward
and acts as the wedge to dilate the cervical canal when labor
begins (
Figure 17–30
). Occasionally, a baby is oriented with
some other part of the body downward (
breech presentation
).
This can require the surgical delivery of the fetus, placenta,
and associated membranes through an abdominal and uterine
incision (
cesarean section
).
At the onset of labor and delivery or before, the amni-
otic sac ruptures, and the amniotic fl uid fl ows through the
vagina. When labor begins in earnest, the uterine contractions
become strong and occur at approximately 10- to 15-min
intervals. The contractions begin in the upper portion of the
uterus and sweep downward.
As the contractions increase in intensity and frequency,
the cervix is gradually forced open (dilation) to a maximum
diameter of approximately 10 cm (4 in). Until this point,
the contractions have not moved the fetus out of the uterus.
Now the contractions move the fetus through the cervix and
vagina. At this time the mother, by bearing down to increase
abdominal pressure, adds to the effect of uterine contractions
to deliver the baby. The umbilical vessels and placenta are
still functioning, so that the baby is not yet on its own, but
within minutes of delivery both the umbilical vessels and the
placental vessels completely constrict, stopping blood fl ow to
the placenta. The entire placenta becomes separated from the
underlying uterine wall, and a wave of uterine contractions
delivers the placenta as the
afterbirth.
Usually, parturition proceeds automatically from begin-
ning to end and requires no signifi cant medical intervention.
In a small percentage of cases, however, the position of the
baby or some maternal complication can interfere with normal
delivery (e.g., breech presentation). The head-fi rst position of
the fetus is important for several reasons: (1) If the baby is not
oriented head fi rst, another portion of its body is in contact
with the cervix and is generally a far less effective wedge.
(2) Because of the head’s large diameter compared with the
rest of the body, if the body were to go through the cervical
canal fi rst, the canal might obstruct the passage of the head,
leading to problems when the partially delivered baby tries to
breathe. (3) If the umbilical cord becomes caught between the
canal wall and the baby’s head or chest, mechanical compres-
sion of the umbilical vessels can result. Despite these potential
problems, however, many babies who are not oriented head
fi rst are born without signifi cant diffi
culties.
What mechanisms control the events of parturition?
1.
The autonomic neurons to the uterus are of little
importance because anesthetizing them does not
interfere with delivery.
2.
The smooth muscle cells of the myometrium have
inherent rhythmicity and are capable of autonomous
contractions, which are facilitated as the muscle is
stretched by the growing fetus.
3.
The pregnant uterus near term and during labor
secretes several prostaglandins (PGE
2
and PGF
2
α
)
that are potent stimulators of uterine smooth muscle
contraction.
4.
Oxytocin,
one of the hormones released from the
posterior pituitary, is an extremely potent uterine
muscle stimulant. It not only acts directly on uterine
smooth muscle but also stimulates it to synthesize the
prostaglandins. Oxytocin is refl exly secreted from the
posterior pituitary as a result of neural input to the
hypothalamus, originating from receptors in the uterus,
particularly the cervix. Also, as noted previously, the
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