BLOC1S3 and osteoporosis: There are several hypotheses regarding causes of pregnancy-associated osteoporosis: 1) increase of parathyroid hormone-related peptide (PTH-rP) secretion by lactation, 2) increase of calcium supply to fetal bone and breast-milk, 3) decrease of estrogen after delivery, 4) osteopenia existing before pregnancy, and 5) heritable factors (2–6).