Adaptation of the circulation to pregnancy occurs via a complicated series of changes not yet completely understood. To define pertinent alterations, echocardiographic measurements of left ventricular function were performed every 4 weeks in 13 normal pregnant women. Six weeks postpartum each women was reexamined; thus each woman served as her own nonpregnant control. Left ventricular dimensions, left venticular wall thickness, shortening fraction, and rate of change of these measurements were recorded. As expected, cardiac output was increased throughout pregnancy. Up to 20 weeks gestation this occurred via an increased heart rate. After 20 weeks gestation stroke volume increased significantly, with 20% at 20--26 weeks up to 30% at term (p < 0.01). With the end-diastolic wall thickness remaining equal, myocardial hypertrophy occurred. This was corroborated by an increase in end-systolic left ventricular wall thickness towards term: from 13.8 mm (S.D. +/- 1.73) in early pregnancy to 16.6 mm (S.D. +/- 1.62) at term, with end-systolic left ventricular dimension unchanged. It was concluded that during pregnancy the mechanism to produce a higher cardiac output shifts from an increase in cardiac frequency to elevation of stroke volume with concomitant myocardial hypertrophy. Due to changes in heart rate and afterload, no conclusions could be drawn regarding myocardial contractility.