Regulation of calcium homeostasis during pregnancy is complex. Clinically significant bone mass loss is infrequent; however a subset of women may develop symptomatic osteoporosis related to pregnancy. Lactation is a period of special risk for bone loss. Whatever the effect of heparin on bone loss, vertebral fractures are rare in women treated with heparin during pregnancy. Low molecular weight heparins may have a less deleterious effect on bone than unfractionated heparin. Women with autoimmune diseases, particularly those with lupus and/or the antiphospholipid syndrome may receive heparin throughout pregnancy. Corticosteroids must be reduced as much as possible in these women, and calcium plus vitamin D are recommended. Finally, indications for heparin use must be clearly justified and advice regarding breastfeeding must be offered.