Women worldwide generally lose weight and body fat during lactation. This loss, although increased by longer, more intensive breastfeeding, is modest and may be reduced by increased food intake and decreased activity. Higher parity and older age are associated with greater weight loss postpartum among poorly nourished women. Well-nourished women or those who breastfeed only for a limited time may not return to their prepregnant weight or body composition by the end of the lactation period. Those who are overweight or obese may have difficulty initiating or maintaining lactation. For the majority of women in the world, lactation is unlikely to represent a threat to their health. To advise women on how to optimize their health and lactational performance, one must consider all of the changes in maternal nutritional status that occur during a reproductive cycle, which may or may not compensate for the modest decreases in body weight associated with lactation.