In utero undernutrition in humans may result in cardiovascular (CV), metabolic, and growth adaptations. In sheep, maternal nutrient restriction during pregnancy, without effects on fetal or birth weight, results in altered CV control in the offspring. Adjustment of gestation length after undernutrition could be a strategy to enhance postnatal health/survival. The aim of this study was to determine in sheep the effect of a 50% reduction in maternal nutrient intake [undernutrition group (U) versus 100%, control group (C)] during 1-31 d of gestation (dGA) on gestation length and offspring size. By 28 dGA, U ewes had gained less weight than C, and twin-bearing ewes had gained less weight than singleton-bearing ewes regardless of group (p<0.05). In different-sex twin pairs, maternal undernutrition resulted in longer gestation compared with C (146.5+/-0.6 versus 144.6+/-0.6 d, p<0.05). Increased weight gain by weaning (20.8+/-0.8 versus 17.9+/-0.8 kg, p<0.05) was observed in U male twins. These findings suggest that the strategy (i.e. growth rate or length of time in utero) adopted by the fetus to enhance immediate survival depends on offspring number and sex. This is likely to reflect the degree of constraint imposed on the fetus.