The hormonal changes of pregnancy alter many of the body systems. Because these normal but complex variations result in visible changes in anatomy, in this chapter, the physiologic changes of pregnancy precede the discussion of anatomy and are briefly summarized here.
■ Estrogen promotes endometrial growth that supports the early embryo. It appears to stimulate marked enlargement of the pituitary gland (by up to 135%) and increased prolactin output from its anterior lobe, which readies breast tissue for lactation. Estrogen also contributes to the hypercoagulable state that puts pregnant women at four to five times higher risk for thromboembolic events, primarily in the venous system.
■ Progesterone levels increase throughout pregnancy, leading to increased tidal volume and alveolar minute ventilation, though respiratory rate remains constant; respiratory alkalosis and subjective shortness of breath result from these changes. Lower esophageal sphincter tone resulting from rising levels of estradiol and progesterone contributes to gastroesophageal reflux. Progesterone relaxes tone in the ureters and bladder, causing hydronephrosis (in the right ureter more than the left) and an increased risk of bacteriuria.
■ Human chorionic gonadotropin (HCG) has five variant subtypes. Two are produced by the placenta and support progesterone synthesis in the corpus luteum, stabilizing the endometrium and effectively preventing loss of the early embryo to menstruation. Serum and urine pregnancy assays test primarily for the two pregnancy-related HCG variants; three isoforms are produced by different cancers and the pituitary gland.
■ Placental growth hormone influences fetal growth and the development of preeclampsia. Placental growth hormone and other hormones have been implicated in insulin resistance after midpregnancy and in gestational diabetes, which carries a lifetime risk of progressing to type 2 diabetes of up to 60%.
■ Thyroid function changes include an increase in thyroid-binding globulin due to rising levels of estrogen and stimulation of thyroid-stimulating hormone (TSH) receptors by HCG. This results in a slight increase, usually in the normal range, in serum concentrations of free T3 and T4, while serum TSH concentrations appropriately decrease. This transient apparent "hyperthyroidism" should be considered physiologic.
■ Relaxin is secreted by the corpus luteum and placenta and is involved in the remodeling of reproductive tract connective tissue to facilitate delivery, increased renal hemodynamics, and increased serum osmolality. Despite its name, relaxin does not affect peripheral joint laxity during pregnancy. Weight gain, especially around the gravid uterus, and shifts in the center of gravity contribute to lumbar lordosis and other musculoskeletal strain.
■ Erythropoietin increases during pregnancy, which raises erythrocyte mass. Plasma volume increases to a greater extent, causing relative hemodilution and physiologic anemia, which can protect against blood loss during birth. Cardiac output increases but systemic vascular resistance decreases, resulting in a net fall in blood pressure, especially during the second trimester and returning to normal by the third trimester.
■ Basal metabolic rate increases 15% to 20% during pregnancy, increasing daily energy demands by an estimated 85, 285, and 475 kcal/d in the first, second, and third trimesters, respectively.
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