The objective of this study was to describe the effects of pregnancy on blood volume and uterine blood flow in sheep, and to test the hypothesis that the effects of periconceptional undernutrition on the late-gestation fetus are mediated by alterations in these parameters. Singleton-bearing ewes that had been undernourished preconception, postconception, both, or neither, underwent estimation of blood volume in mid and late gestation, and measurement of uterine blood flow in late gestation. Seven non-pregnant ewes were also studied. Pregnancy resulted in a 31% greater red cell volume in mid-gestation (21.0+/-1.3 vs 16.1+/-0.8ml/kg, p<0.05), but no significant change in plasma or blood volume. However maternal blood volume was correlated with uterine blood flow (r(2)=0.22, p=0.05) and fetal size (r(2)=0.20, p=0.02). Uterine blood flow was 13% greater in the undernourished groups than controls (1847+/-100 vs 1641+/-79ml/min, p<0.01). The large increase in maternal blood volume integral to a successful human pregnancy was not present in sheep. The increased uterine blood flow after periconceptional undernutrition suggests that nutritional signals before and in early pregnancy influence fetal nutrient supply in late gestation.