Twelve normal-weight and 12 underweight women were compared to test whether fetal growth retardation in underweight gravidas is related to inadequate maternal hemodynamic adjustments. Plasma volume (+/- standard error) was 3227 +/- 103 mL in normal-weight and 2731 +/- 84 mL in underweight women (P < .002). Cardiac output was 6340 +/- 167 mL/minute in controls and 5689 +/- 213 mL/minute in underweight women (P < .03). Total peripheral vascular resistance was lower in controls than in underweight subjects (1025 +/- 31 versus 1198 +/- 58 dyne/second/cm5). Mean birth weight was 2837 +/- 125 g in underweight women and 3362 +/- 106 g in controls (P < .005). Similarly, placental weight was reduced in the underweight group. All infants delivered by control mothers had a normal birth weight, whereas six infants from underweight gravidas were growth-retarded. In all cases combined, maternal plasma volume correlated significantly with both birth weight (r = 0.6, P < .002) and placental weight (r = 0.56, P < .01); total peripheral vascular resistance also correlated significantly and inversely with newborn weight and placental weight. Cardiac output correlated only with placental weight (r = 0.54, P < .02). These results are consistent with the hypothesis that underweight mothers are at higher risk of fetal growth retardation because of a smaller plasma volume and lower cardiac output.
(C) 1992 The American College of Obstetricians and Gynecologists