Protein deficiency during pregnancy affects fetal development. The critical period, when the fetus is most susceptible to maternal protein deficiency and its effect on neonatal growth, is unknown. Therefore, we studied the effect of a protein-restricted diet during early and late pregnancy and throughout pregnancy on growth of pigs from birth to market weight. Sows were fed a control (13% protein) or protein-restricted (0.5% protein) diet throughout pregnancy or protein-restricted diet from d 1 to 44, then control diet to term or control diet from d 1 to 81, then the protein-restricted diet to term. In Experiment 1, birth weights were measured, and 12 pigs/diet group were weaned at 4 wk and raised to market weight. Feeding the protein-restricted diet throughout pregnancy reduced birth and slaughter weights, whereas the control followed by protein-restricted and protein-restricted followed by control diets reduced only birth weight relative to controls. Indices of carcass lean were reduced in the protein-restricted piglets, with carcass fat not affected. In Experiment 2, control and control-protein-restricted litters were reduced to six piglets and 3/litter cross-fostered to a sow of the other treatment group. After weaning at 4 wk, 4 piglets/group were individually fed to 8 wk. The control and control followed by protein-restricted diet fed piglets had similar weights at birth, but piglets raised by a control-protein-restricted sow tended to weight less at weaning than their littermates raised by a control sow. After weaning, these piglets had greater feed intakes relative to other groups and there were no weight differences by 8 wk.(ABSTRACT TRUNCATED AT 250 WORDS)