The maternal heart rate increases by 10–15 beats per minute, from 70 to 85 beats per minute. In addition to the increase in heart rate, there is an increase in the occurrence of arrhythmias, which are usually benign. Total blood volume increases by 40–50% above pre-pregnancy values. The plasma volume expands proportionately more than the increase in red blood cell mass, accounting for the physiologically lower hematocrit during pregnancy. The maximum increase in plasma volume occurs between 20 and 30 weeks’ gestation, a time of increased risk for mothers with heart disease sensitive to volume overload. By 30 weeks’ gestation, the cardiac output will be 30–50% higher than before pregnancy. Seventeen percent of the cardiac output goes to the uterus.

Maternal body position, labor and anesthesia can affect the cardiovascular status. Prolonged standing causes venous pooling and decreases venous return to the heart, which in turn decreases cardiac output and may provoke syncope. Supine hypotension may occur when the gravid uterus compresses the vena cava and impairs venous return to the heart, causing a fall in cardiac output and blood pressure.

Pain from uterine contractions causes maternal tachycardia, which may have unfavorable hemodynamic effects because it reduces the diastolic filling time. Effective analgesia will blunt this effect. For nearly all types of cardiac disease, epidural anesthesia is preferred for either vaginal or cesarean deliveries. However, it must be administered with care to avoid hypotension. The narcotic epidural or combined narcotic epidural and spinal may be used in patients who are very sensitive to changes in systemic resistance.

During the second stage of labor, the maternal Valsalva maneuver from bearing-down efforts decreases venous blood return to the heart because it increases intrathoracic pressure. There is a simultaneous increase in peripheral resistance as well but because of the decreased cardiac output, the blood pressure does not increase. When the straining is stopped, there is a rapid increase in cardiac output and blood pressure. The consequences of the Valsalva maneuver may affect cardiac conditions that are sensitive to decreased filling pressure or increased systemic resistance.

In the immediate postpartum period there is an abrupt increase in cardiac output by 60%. This is due to re-entry of blood, which was previously diverted to the uterus or pooled in the partially obstructed venous circulation of the lower extremities, into the central circulation. Stroke volume is increased, and there is a reflexive fall of heart rate.

The normal physiologic changes associated with pregnancy are well tolerated by women with normal hearts. Patients with cardiac disease who are unable to tolerate these changes may decompensate and develop congestive heart failure.

Normal pregnant women frequently have symptoms and signs that could be interpreted as indicative of heart disease in the nonpregnant state. Dyspnea occurs in up to 60% of pregnant women. It is usually described as “a sense of not being able to breathe quite deeply enough to get all the air one needs.” Increased fatigue is a common complaint, especially during the first and last trimesters. Lower-extremity edema is commonly seen in the third trimester.