Periconceptional undernutrition alters fetal growth and development. However, there are no data on separate effects of undernutrition before and after conception and few on underlying mechanisms. We determined the effects of mild periconceptional undernutrition on late gestation fetal growth, glucose-insulin axis, and maternal and fetal hypothalamic-pituitary-adrenal axes. Ewes were undernourished for 60 d before conception, 30 d after, or both, compared with well-nourished controls. Undernutrition before conception resulted in smaller, slower-growing fetuses with relatively larger placentae. Ewes that gained weight before, but lost weight after mating, or vice versa, had the smallest fetuses. Fetuses of ewes undernourished only before conception grew more slowly following instrumentation, and fetuses in both preconception undernutrition groups slowed their growth with a maternal fast. The fetal glucose-insulin axes and maternal and fetal hypothalamic-pituitary-adrenal axis were not different among groups. Maternal undernutrition at different periods around conception has different effects on fetal growth trajectory that are not reflected in size in late gestation. Preconceptional undernutrition alone alters fetal growth responses to late gestation stressors, suggesting that maternal nutrition is important at both times, and that fetal effects are neither due solely to substrate limitation, nor to excess fetal glucocorticoid exposure at the time of undernutrition.