The authors report two cases of osteoporosis, complicated by collapse of corpus vertebrae, which occurred during the last three months of pregnancy. Results of the study of phosphorus and calcium balance are not different from those found in other types of osteoporosis occurring in young adults. There is no antecedent osteopathy. Calciuria is normal. From analysis of previously published studies addressing the metabolism of phosphorus and calcium during pregnancy, a few pathogenic hypotheses for this rare aspect of osteoporosis are discussed. Special attention is given to calcitonin activity during late pregnancy.