A major assumption in current dietary advice for pregnancy is that the amount of energy expended on tissue maintenance increases by 36 000 kcal over the whole gestation period. In a group of rural Gambian women engaged in subsistence farming, the increase was much smaller and depended on maternal dietary status. In women having the customarily low food intake the net extra cost of tissue maintenance was just 1000 kcal; even in those given a dietary supplement in amounts previously shown to increase birthweight it was only 13 000 kcal. This finding partly explains why mothers, particularly those in the Third World, can apparently accomodate pregnancy without substantially increasing their dietary energy intake. The findings are of importance in the formulation of realistic dietary health strategies for such communities.