During the first 10-12 weeks of pregnancy, the
ovary's corpus luteum produces most of the progesterone. After
this time, the placenta produces enough progesterone, allowing
the corpus luteum to decrease production.
The precursors of progesterone do not come from
the fetus, as they do with estrogen. The precursors are synthesized from maternal cholesterol.
Some of the actions of progesterone can be
thought of as:
- "balancing" the
action of estrogen. That is, if one is
"pushing", then the other is
- acting together
with estrogen for the same effect.
- having an apparently unrelated
action to estrogen.
For example, progesterone balances
- It reduces the "tone" of many of
the body tissues, including the uterus. Remember,
estrogen increases uterine muscle contractility. This
increases the strength, tone, and ability of the uterine
muscles to contract in preparation for delivery.
Progesterone has the opposite effect by inhibiting
myometrium contractility. Contractions are suppressed by
progesterone. Contractions which normally occur 3-4 times
per minute during ovulation are inhibited by
progesterone. This allows the blastocyst to implant and
develop and protects against expulsion of the developing
pregnancy. After the 12th week, there are
normally irregular and painless contractions lasting a
few seconds. Progesterone declines towards the time of
delivery, leaving estrogen to increase the uterine
Related to this action, progesterone
- Decreases sensitivity to oxytocin.
Oxytocin (a hormone from the posterior pituitary)
increases the frequency and strength of uterine
contractions. During late pregnancy, the progesterone
level may drop to 250 mg/day. As progesterone decreases,
sensitivity is increased to oxytocin. This action is
important to keep in mind when high risk events (either
maternal or fetal) influence a provider's decision to
induce labor in a preterm fetus. Administering
intravenous oxytocin (pitocin, "syntocinon") in
large quantities may not be able to overcome
progesterone's influence and the woman "fails"
to go into labor.
responsible for making uterine muscle more contractile, while
estrogen causes it to relax.
TRUE or FALSE
Together with estrogen,
- Develops the uterine endometrium, or
lining of the uterus, to prepare it for implantation.
When levels of progesterone and estrogen are high enough
to maintain the endometrium, the blastocyst is implanted.
- Affects the metabolism, promoting fat
- Maternal stores of body fat
provide a reserve of energy for pregnancy and
- Prepares breasts for lactation. Though
estrogen is responsible for the increase in ductal
growth, progesterone stimulates the development of lobes
and lobules in the breast.
Progesterone can be thought of as the hormone
responsible for many of the body tissues to "relax" or
decrease tone. It is responsible for the decrease in tone of the:
- Vascular system. It relaxes blood vessel
walls and decreases vascular resistance. It contributes
to vascular dilation and edema, especially lower limb
edema and varicosities. An increase
in hemorrhoids can also be attributed to progesterone.
- GI tract. Relaxation of the GI tract may
contribute to constipation and heartburn. The decreased
motility of the gallbladder may increase the incidence of
- Renal system. This may contribute to
urinary stasis and ureter dilation. This may be one
contributing factor for bladder infections.
- Smooth muscle of respiratory tract,
therefore decreasing airway resistance.
- Musculoskeletal system, relaxing muscles,
ligaments and joints, and sometimes decreasing muscle
- Progesterone works together with
relaxin to affect some of the musculoskeletal
changes. The pregnant woman may be surprised at
her inability to pick up a heavy item with one
hand, or she may have to adjust to changes in
posture. The relaxation in chest ligaments
actually causes an increase in chest size, with
larger thoracic cage dimensions.
Much of these actions will be explained in
detail in later modules, but other activities that progesterone
is involved with include:
- Progesterone has a very interesting direct
action on the mother's respiratory center (medulla
- It increases sensitivity to carbon
dioxide. To say it another way: The respiratory
center can't tolerate the usual amount of carbon
dioxide in the blood. This accounts for the feeling
of shortness of breath, dyspnea, or heightened
awareness of the need to breathe reported by many
pregnant women. The pregnant woman is actually forced to hyperventilate, and "blow off" more
carbon dioxide than she normally would. As a
result, the removal of carbon dioxide from the
fetus's bloodstream is facilitated!
- In other words: In a blood gas
analysis, the normal nonpregnant woman would be
expected to have a pCO2 of 35-45 mmHg.
- The normal pregnant woman would be
expected to have a pCO2 of 27-32 mmHg.
- Affects the metabolism, increasing body
- Progesterone raises body
temperature about .5 degrees Centigrade (0.9
- Inhibits the action of prolactin
- As a result, milk production
begins only after delivery of the placenta.
- Stimulates the loss of
sodium in the urine.
- This in turn, increases
aldosterone production to conserve sodium.
- Affects the central nervous system
- Commonly produces maternal
complaints of being tired.
responsible for a decreased amount of carbon dioxide in the blood
of pregnant women.
TRUE or FALSE
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