Progesterone

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 broken down from the mother's 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 "pulling".
• acting together with estrogen for the same effect.
• having an apparently unrelated action to estrogen.

For example, progesterone balances estrogen when:

• 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 12^th 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 contractility.

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.

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Progesterone is responsible for making uterine muscle more contractile, while estrogen causes it to relax.

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Together with estrogen, progesterone:

• 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 deposition.
  • Maternal stores of body fat provide a reserve of energy for pregnancy and lactation.
• 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 gallbladder disease.
• 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 strength.
  • 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 oblongata).
  • 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 temperature
  • Progesterone raises body temperature about .5 degrees Centigrade (0.9 degrees F.)
• 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.”

WebMd mentions the action of progesterone, in this article about the kidneys and pregnancy. Look for the answer to this question.

The action of progesterone on the vessels of the kidney tends to make the kidneys larger or smaller during pregnancy?

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Progesterone is responsible for a decreased amount of carbon dioxide in the blood of pregnant women.

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