Each year, approximately 4 million births occur in the United States, and more than 130 million births occur worldwide. A significant proportion of births are complicated by medical disorders. Advances in medical care and fertility treatment have increased the number of women with serious medical problems who attempt to become pregnant. Medical problems that interfere with the physiologic adaptations of pregnancy increase the risk for poor pregnancy outcome; conversely, in some instances, pregnancy may adversely impact an underlying medical disorder.
(See also Chap. 271) In pregnancy, cardiac output increases by 40%, with most of the increase due to an increase in stroke volume. Heart rate increases by ~10 beats/min during the third trimester. In the second trimester, systemic vascular resistance decreases, and this decline is associated with a fall in blood pressure. During pregnancy, a blood pressure of 140/90 mmHg is considered to be abnormally elevated and is associated with an increase in perinatal morbidity and mortality. In all pregnant women, the measurement of blood pressure should be performed in the sitting position, because the lateral recumbent position may result in a lower blood pressure. The diagnosis of hypertension requires the measurement of two elevated blood pressures at least 4 h apart. Hypertension during pregnancy is usually caused by preeclampsia, chronic hypertension, gestational hypertension, or renal disease.
Approximately 5–7% of all pregnant women develop preeclampsia, the new onset of hypertension (blood pressure >140/90 mmHg) and proteinuria (either a 24 h urinary protein >300 mg/24 h, or a protein-creatinine ratio ≥0.3) after 20 weeks of gestation. Recent revisions to the diagnostic criteria include: proteinuria is no longer an absolute requirement for making the diagnosis; the terms mild and severe preeclampsia have been replaced; and the disease is now termed preeclampsia either with or without severe features and fetal growth restriction is no longer a defining criterion for preeclampsia with severe features. Although the precise pathophysiology of preeclampsia remains unknown, recent studies show excessive placental production of antagonists to both vascular endothelial growth factor (VEGF) and transforming growth factor β (TGF-β). These antagonists to VEGF and TGF-β disrupt endothelial and renal glomerular function resulting in edema, hypertension, and proteinuria. The renal histological feature of preeclampsia is glomerular endotheliosis. Glomerular endothelial cells are swollen and encroach on the vascular lumen. Preeclampsia is associated with abnormalities of cerebral circulatory autoregulation, which increase the risk of stroke at mildly and moderately elevated blood pressures. Risk factors for the development of preeclampsia include nulliparity, diabetes mellitus, a history of renal disease or chronic hypertension, a prior history of preeclampsia, extremes of maternal age (>35 years or <15 years), obesity, antiphospholipid antibody syndrome, and multiple gestation. Low-dose aspirin (81 mg daily, initiated at the end of the first trimester) modestly reduces the risk of preeclampsia in pregnant women at high risk of developing the disease.