Chapter 28
Pregnancy and Human Development
1085
28
release
prostaglandins
(pros
0
tah-glan
9
dinz) (Figure 28.17) which,
like estrogens, stimulate the synthesis of more gap junctions in
uterine smooth muscle. Both hormones are powerful uterine
muscle stimulants, and since the myometrium is now highly
sensitive to oxytocin, contractions become more frequent and
more vigorous. While elevated levels of oxytocin and prosta-
glandins sustain labor once it begins, many studies indicate that
it is the prostaglandins (acting as paracrines) that actually trig-
ger the rhythmic expulsive contractions of true labor, and play a
major role in the thinning and so±ening of the cervix just before
and during labor. At this point, the increasing cervical distension
activates the mother’s hypothalamus, which signals for oxytocin
release by the posterior pituitary.
Once the hypothalamus is involved, several
positive feedback
mechanisms
involving prostaglandins and oxytocin are pro-
pelled into action—greater distension of the cervix causes the
release of more oxytocin, which causes greater contractile force,
and so on (Figure 28.17). Tese expulsive contractions are aided
by the fact that
fetal fibronectin
, a natural “stickum” (adhesive
protein) that binds the fetal and maternal tissues of the placenta
together throughout pregnancy, changes to a lubricant just be-
fore true labor begins.
As mentioned earlier, prostaglandins are essential for initi-
ating labor in humans, and interfering with their production
will hinder onset of labor. For example, antiprostaglandin drugs
such as ibuprofen can inhibit the early stages of labor and such
drugs are used occasionally to prevent preterm births.
Stages of Labor
Labor includes the dilation, expulsion, and placental stages il-
lustrated in
Figure 28.18
.
Homeostatic Imbalance
28.2
A dangerous complication of pregnancy called
preeclamp-
sia
results in an insufficient placental blood supply, which can
starve a fetus of oxygen. Te pregnant woman becomes edema-
tous and hypertensive, and proteinuria occurs. Tis condition,
which affects one in 10 pregnancies, is believed to be due to
immunological abnormalities in some cases, because its occur-
rence seems to be positively correlated with the number of fetal
cells that enter the maternal circulation.
Check Your Understanding
15.
What causes the difficult breathing that some women
experience during pregnancy? What causes the waddling
gait seen in some?
16.
What is the cause of morning sickness?
17.
What is the role of the hormone hPL?
For answers, see Appendix H.
Parturition (Birth)
Explain how labor is initiated, and describe the three stages
of labor.
Parturition
(par
0
tu-rish
9
un; “bringing forth young”) is the cul-
mination of pregnancy—giving birth to the baby. It usually oc-
curs within 15 days of the calculated due date (280 days from
the last menstrual period). Te series of events that expel the
infant from the uterus are collectively called
labor
.
Initiation of Labor
Several events and hormones interlock to trigger labor. During
the last few weeks of pregnancy, estrogens reach their highest
levels in the mother’s blood. Studies indicate that the fetus de-
termines its own birth date. Rising levels of fetal adrenocortical
hormones (especially cortisol) late in pregnancy are believed to
stimulate the placenta to release such large amounts of estro-
gens. In addition, increased production of
surfactant protein A
(
SP-A
) by the fetal lungs in the weeks before delivery appears to
trigger an inflammatory response in the cervix that stimulates
its so±ening in preparation for labor.
Te rise in estrogens has three important consequences.
(1) It stimulates the myometrial cells of the uterus to form abun-
dant oxytocin receptors
(Figure 28.17)
. (2) It promotes forma-
tion of gap junctions between the uterine smooth muscle cells.
(3) It antagonizes progesterone’s quieting influence on uterine
muscle. As a result, the myometrium becomes increasingly ir-
ritable, and weak, irregular uterine contractions begin to oc-
cur. Tese contractions, called
Braxton Hicks contractions
, have
caused many women to go to the hospital, only to be told that
they were in
false labor
and sent home.
As birth nears, two more chemical signals cooperate to convert
these false labor pains into the real thing. Certain fetal cells begin
to produce
oxytocin
(ok
0
sĭ-to
9
sin), which causes the placenta to
Estrogen
Start
from
placenta
Induces oxytocin
receptors on uterus
Oxytocin
from fetus
and mother's
posterior pituitary
Stimulates uterus
to contract
Positive feedback
Stimulates
placenta to release
Stimulate more
vigorous contractions
of uterus
Prostaglandins
(+)
(+)
Figure 28.17
Hormonal induction of labor.
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