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Chapter: Obstetrics and Gynecology: Maternal-Fetal Physiology

Maternal Physiology: Other Maternal Systems

As pregnancy advances, a compensatory lumbar lordosis (anterior convexity of the lumbar spine) is apparent.

Other Maternal Systems




As pregnancy advances, a compensatory lumbar lordosis (anterior convexity of the lumbar spine) is apparent. This change is functionally useful, because it helps keep the woman’s center of gravity over the legs; otherwise, the enlarging uterus would shift it anteriorly. However, as a result of this change in posture, virtually all women com-plain of low back pain during pregnancy. Increasing pres-sure caused by intra-abdominal growth of the uterus may result in an exacerbation of hernia defects, most commonly seen at the umbilicus and in the abdominal wall (diastasis recti, a physiologic separation of the rectus abdominus mus-cles). Beginning early in pregnancy, the effects of relaxin and progesterone result in a relative laxity of the ligaments. The pubic symphysis separates at approximately 28 to 30 weeks. Patients often complain of an unsteady gait and may fall more commonly during pregnancy than during the nonpregnant state, as a result of both these changes and an altered center of gravity.


To provide for adequate calcium supplies to the fetal skeleton, calcium stores are mobilized. Maternal serum ionized calcium is unchanged from the nonpregnant state, but maternal total serum calcium decreases. There is a sig-nificant increase in maternal parathyroid hormone, which maintains serum calcium levels by increasing absorption from the intestine and decreasing the loss of calcium through the kidney. The skeleton is well-maintained despite these elevated levels of parathyroid hormones. This may be because of the effect of calcitonin. Although the rate of bone turnover increases, there is no loss of bone density during a normal pregnancy if adequate nutrition is supplied. 



Pregnancy induces several characteristic changes in the appear-ance of the maternal skin. Although the exact etiology ofthese changes has not been established, hormonal influ-ences appear to predominate.

Vascular spiders (spider angiomata) are most com-mon on the upper torso, face, and arms. Palmar erythema occurs in more than 50% of patients. Both are associated with increased levels of circulating estrogen and regress after delivery. Striae gravidarum occur in more than half of pregnant women and appear on the lower abdomen, breasts, and thighs. Initially, striae can be either purple or pink; eventually, they become white or silvery. These striae are not related to weight gain, but are solely the result of the stretching of normal skin. There is no effective therapy to prevent these “stretch marks,” and once they appear, they cannot be eliminated.


Pregnancy may produce characteristic hyperpigmen-tation, which is believed to be the result of elevated levelsof estrogen and a melanocyte-stimulating hormone and a cross-reaction with the structurally similar hCG. Hyperpigmentation commonly affects the umbilicus and perineum, although it may affect any skin surface. The lower abdomen linea alba darkens to become the linea nigra. The “mask of pregnancy,” or chloasma (melasma), is also common and may never disappear completely. Skin nevi can increase in size and pigmentation, but resolve after pregnancy; however, removal of rapidly changing nevi is recommended during pregnancy, because of the risk of malignancy. Eccrine sweating and sebum production increase during normal pregnancy, with many patients complaining of acne.


Hair growth during pregnancy is maintained, although there are more follicles in the anagen (growth) phase and fewer in the telogen (resting) phase. Late in pregnancy, the number of hairs in telogen is approximately half of the normal 20%, so that postpartum, the number of hairs entering telogen increases; thus, there is significant hair loss 2 to 4 months after pregnancy. Hair growth typically returns to normal 6 to 12 months after delivery. Patients are often concerned about this “hair loss,” until they are reassured that it is transient and that hair growth will renew.



The effects of pregnancy on the vulva are similar to the effects on other skin. Because of an increase in vascularity, vulvar varicosities are common and usually regress after delivery. An increase in vaginal transudation as well as stim-ulation of the vaginal epithelium results in a thick, profuse vaginal discharge, called leukorrhea of pregnancy. The epithelium of the endocervix everts onto the ectocervix, which is associated with a mucous plug.


During pregnancy, the uterus undergoes an enor-mous increase in weight from the 70-g nonpregnant size to approximately 1100 g at term, primarily through hypertro-phy of existing myometrial cells. After pregnancy, the uterus returns to an only slightly increased size as the actual num-ber of cells comprising it are minimally increased. Similarly, the uterine cavity enlarges to a volume of up to as much as 5 liters, compared to less than 10 mL in the nongravid state.




The breasts increase in size during pregnancy, rapidly in the first 8 weeks and steadily thereafter. In most cases, the totalenlargement is 25% to 50%. The nipples become larger and more mobile and the areola larger and more deeply pigmented, with enlargement of the Montgomery glands. Blood flow to the breasts increases as they change to sup-port lactation. Some patients may complain of breast or nipple tenderness and a tingling sensation. Estrogen stim-ulation also results in ductal growth, with alveolar hyper-trophy being a result of progesterone stimulation. During the latter portion of pregnancy, a thick, yellow fluid can be expressed from the nipples. This is colostrum, more com-mon in parous women. Ultimately, lactation depends on synergistic actions of estrogen, progesterone, prolactin, human placental lactogen, cortisol, and insulin.




The most common visual complaint during pregnancy is blurred vision. This visual change is primarily caused by increased thickness of the cornea associated with fluid reten-tion and decreased intraocular pressure. These changes are manifest in the first trimester and regress within the first 6 to 8 weeks postpartum. Therefore, changes in correc-tive lens prescriptions should not be encouraged during pregnancy.


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