Estrogen

The pregnant woman at term produces more estrogen in one day, than a nonpregnant woman produces in 3 years! Estrogen is responsible for enormous changes during pregnancy.

The most common estrogens in the human are:

• estradiol (mostly secreted by the ovaries),
• estrone and
• estriol (mostly secreted by the placenta). During pregnancy, there is a large increase in the amount of estriol in relationship to estrone and estradiol.

The fetal adrenal glands and the placenta interact in order to produce estrogen in appropriate amounts.

• The adrenal glands of the fetus provide essential precursors for the production of estriol in the placenta, therefore a living, healthy fetus and placenta are needed to produce ideal estrogen levels. This is an example of the "placenta/fetal unit" at work.
• The maternal liver conjugates the estriol, and it is excreted in her urine.
• The level of urinary estriol increases gradually until the12th week of pregnancy, and then increases more rapidly until term. Measurement of estriols in the maternal urine has historically been used as an indicator of fetal and placental well being.

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The fetus and the placenta must interact for appropriate production of estrogen.

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Estrogen has so many functions during pregnancy. The following is only a partial list, but it should cover the most relevant. To organize these functions, they are separated here between those that "build tissue," and those that "change tissue" in some other way.

Estrogen has a major role during pregnancy of building tissue. It accomplishes this by either increasing the size or the number of tissues, vessels, and blood cells.

• It is responsible for the proliferation of the uterine endometrium. This increase in the depth and consistency of the uterine lining prepares it for implantation.
• Estrogen is a major factor in the increased size of the uterus and thickening of the uterine wall. During the pregnancy, the uterus increases in size to accommodate the developing pregnancy. It does this through hyperplasia (increase in the number of cells), hypertrophy (increase in the size of cells) of muscle cells, and stretching of the uterus.
  • The uterus grows from 60 grams in weight to 1200 grams (20 times it's nonpregnant weight), with a volume increasing from 10 cc to 2-10 liters of fluid.
• In addition there is an increase in the size of the external genitalia. The vaginal walls are thick and easily distensible. The genitalia is also very vascular.
• Estrogen is responsible for an increased blood, lymphatics and nerve supply to the uterus, and throughout the body. The considerable increase in the number and size of vessels in the uterine vascular bed causes a decrease in the uterine vascular resistance. As a result, blood flows to and from the uterus more freely.
  • Uterine blood flow increases from 50cc per minute at 10 weeks to 500cc per minute at term.
  • Uterine veins may dilate up to 60 times larger than prepregnant states, in order to produce adequate drainage for the large uteroplacental blood flow.
• Estrogen also contributes to the increase in breast size, especially the alveolar ductile tissue. The numbers and size of ducts and lobes increase, vascularity increases, and the nipples and areolae become larger. Montgomery's glands appear as small elevations in areolae. Increase ductal growth is estrogen's responsibility, while increased lobular development is attributed to progesterone.

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Estrogen is responsible for the increased size of the uterus, and more blood vessels both to and from the uterus.

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Another function of estrogen is change in the character of tissue.

• There is a change in composition of the blood.
  • Though there is a decrease in the total plasma proteins, there is an increase in serum-binding proteins. Estrogen increases the availability of body proteins, maintains a positive nitrogen balance and ensures there is adequate protein for fetal growth. It also causes increased deposition of fat.
  • There is a significant increase in the number of white blood cells in the blood, ("leukocytosis of pregnancy"), as may occur during acute infection or inflammation.
  • There is also an increase in fibrinogen concentration and decrease in fibrinolytic activity. As a result, blood clots or coagulates more readily. This decreases the risk of hemorrhage, but also increases the risk of blood clots or thrombosis.
• There is a softening of connective and collagen tissues. Estrogen, along with the hormone relaxin, relaxes pelvic joints and ligaments, and increases mobility of nipple tissue. In late pregnancy, fibers of the cervix are swollen and loosely connected to each other. This is the “ripening” that prepares the cervix to dilate.
  • Endocrine factors, including estrogen, are also thought to be responsible for changes in the blood vessel walls. Blood vessels become more permeable during pregnancy. The result is frequently leakage into dependent tissues, or "swollen ankles."
• Estrogen causes the kidney tubules to retain more sodium and water. This leads to a significant increase in blood volume.
• Uterine tissue becomes more elastic and contractile, under the influence of estrogen. These features help the uterus accommodate the growing fetus and prepare for the contractions that bring about childbirth.
• Estrogen is also responsible for skin changes (striae gravidarum, telangiectases, palmar erythema, hyperpigmentation). Though we won't go into detail about these changes in this module, we should note that many of them appear due to an increased number and size of blood vessels.
• Estrogen also causes increased acidity (lower pH) in the vagina.
  • The mechanism for lower pH is attributed to a general increase in metabolic activity under the influence of estrogen. The glycogen in the vaginal epithelium breaks down and results in increased lactic acid. The lower pH (3.5-6) is thought to ward off infection to some degree and alter the growth of pathogenic bacteria in the vagina.
• There is a decrease in gastric secretion of hydrochloric acid and pepsin under the influence of estrogen. This leads to decreased fat absorption and increasing digestive upsets and nausea. On the other hand, ulcers may improve!

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Estrogen causes a decreased ability for the blood to clot in a pregnant woman.

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