Effect of Labor on Maternal Physiology

Effect of Labor on Maternal Physiology

During intense painful contractions, maternal min-ute ventilation may increase up to 300%. Oxygen consumption also increases by an additional 60% above third-trimester values. With excessive hyper-ventilation, Paco₂ may decrease below 20 mm Hg. Marked hypocapnia can cause periods of hypoven-tilation and transient maternal and fetal hypoxemia between contractions. Excessive maternal hyper-ventilation also reduces uterine blood flow and pro-motes fetal acidosis.

Each contraction places an additional burden on the heart by displacing 300–500 mL of blood from the uterus into the central circulation (analogous to an autotransfusion). Cardiac output rises 45% over third-trimester values. The greatest strain on the heart, however, occursimmediately after delivery, when intense uterine contraction and involution suddenly relieve inferior vena caval obstruction and increase cardiac output as much as 80% above late third trimester values.