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Medicine LibreTexts

27.10C: Physiological Changes

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    8295
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    Maternal physiological changes in pregnancy are entirely normal and serve as adaptations to better accommodate embryonic/fetal development.

     

    LEARNING OBJECTIVES

     

    Describe the physiological changes a woman undergoes during pregnancy

     

    KEY TAKEAWAYS

    Key Points

     

    • Women undergo several changes during pregnancy, including cardiovascular, hematologic, metabolic, renal, and respiratory changes that provide adequate nutrition and gas exchange for the developing fetus.
    • Progesterone and estrogen levels rise continually through pregnancy, together with blood sugar, breathing rate, and cardiac output.
    • The body’s posture changes during pregnancy to accommodate the growing fetus and the mother will experience weight gain.
    • Breasts grow and change in preparation for lactation once the infant is born. Once lactation begins, the woman’s breasts swell significantly and can feel achy, lumpy, and heavy (engorgement). This is relieved by nursing the infant.
    • Plasma and blood volume increase over the course of the pregnancy and lead to changes in heart rate and blood pressure. Women may also have a higher risk of blood clots, especially in the weeks following labor.

     

    Key Terms

     

    • human placental lactogen: Also called human chorionic somatomammotropin, this is a polypeptide placental hormone. Its structure and function is similar to that of human growth hormone. It modifies the metabolic state of the mother during pregnancy to facilitate the energy supply of the fetus.
    • human chorionic gonadotropin: A peptide hormone produced during pregnancy that prevents the breakdown of the corpus luteum and maintains progesterone production.
    • progesterone: A steroid hormone, secreted by the ovaries, whose function is to prepare the uterus for the implantation of a fertilized ovum and to maintain pregnancy.

    Maternal physiological changes in pregnancy are the normal adaptations that a woman undergoes during pregnancy to better accommodate the embryo or fetus, and include cardiovascular, hematologic, metabolic, renal, and respiratory changes. The female body must change its physiological and homeostatic mechanisms in pregnancy to ensure proper fetal development. Increases in blood sugar, breathing, and cardiac output are all required.

    Hormonal Changes

    Pregnant women experience adjustments in their endocrine system. Levels of progesterone and estrogens rise continuously throughout pregnancy to suppress the hypothalamic axis and, subsequently, the menstrual cycle.

    Estrogen produced by the placenta is associated with fetal well being. Women also experience an increase in human chorionic gonadotropin (β-hCG), which is produced by the placenta and maintains progesterone production by the corpus luteum.

    The increase in progesterone production primarily functions to relax smooth muscles. Prolactin levels increase due to maternal pituitary gland enlargement that mediate a change in the structure of the mammary gland from ductal to lobular-alveolar.

    Parathyroid hormone increases and leads to increased calcium uptake in the gut and reabsorption by the kidney. Adrenal hormones such as cortisol and aldosterone also increase.

    Human placental lactogen (HPL) is produced by the placenta, stimulating lipolysis and fatty acid metabolism by the woman and conserving blood glucose for use by the fetus. It can also decrease maternal tissue sensitivity to insulin and result in gestational diabetes.

    Weight Changes

    One of the most noticeable alterations in pregnancy is the gain in weight. The enlarging uterus, the growing fetus, the placenta and liquor amnii, and the acquisition of fat and water retention, all contribute to weight gain.

    The weight gain varies and can be anywhere from five pounds (2.3 kg) to over 100 pounds (45 kg). In the U.S., the doctor-recommended weight gain range is 25 pounds (11 kg) to 35 pounds (16 kg), less if the woman is overweight, more (up to 40 pounds 18 kg) if the woman is underweight.

    This photograph is a silhouette of a nude, pregnant woman.

     

    Pregnancy: During pregnancy, a woman gains weight and her breasts enlarge.

    A woman’s breasts grow during pregnancy, usually one to two cup sizes, but possibly larger. A woman who wore a C cup bra prior to her pregnancy may need to buy an F cup or larger bra while nursing. A women’s torso also grows and her bra band size may increase one or two sizes.

    Once the baby is born (about 50 to 73 hours after birth), the mother will experience her breasts filling with milk, at which point changes in the breast happen very quickly. Once lactation begins, the woman’s breasts swell significantly and can feel achy, lumpy, and heavy (engorgement). Her breasts may increase again in size. Individual breast size can vary daily or for longer periods depending on how much the infant nurses from each breast.

    Circulatory Changes

    Plasma and blood volume slowly increase by 40–50% over the course of the pregnancy (due to increased aldosterone) to accommodate the changes, resulting in an increase in heart rate (15 beats/min more than usual), stroke volume, and cardiac output. Cardiac output increases by about 50%, primarily during the first trimester.

    The systemic vascular resistance also drops due to the smooth muscle relaxation and overall vasodilation caused by elevated progesterone, leading to a fall in blood pressure. Diastolic blood pressure consequently decreases between 12–26 weeks, and increases again to pre-pregnancy levels by 36 weeks.

    Edema (swelling) of the feet is common during pregnancy, partly because the enlarging uterus compresses veins and lymphatic drainage from the legs.

    A pregnant woman will also become hypercoagulable, leading to increased risk for developing blood clots and embolisms due to increased liver production of coagulation factors. Women are at highest risk for developing clots (thrombi) during the weeks following labor.

    Clots usually develop in the left leg or the left iliac venous system because the left iliac vein is crossed by the right iliac artery. The increased flow in the right iliac artery after birth compresses the left iliac vein leading to an increased risk for thrombosis (clotting) that is exacerbated by a lack of ambulation (walking) following delivery. Both underlying thrombophilia and caesarean section can further increase these risks.