The objective of this study was to examine the influence of twin pregnancy on calcium metabolism, including bone turnover markers and calcium-regulating factors, by comparison between singleton pregnancy and twin pregnancy in women during pregnancy and puerperium in cross-sectional and longitudinal studies.Women with singleton and twin pregnancies were recruited from the outpatient clinic of Tokushima University Hospital. In both cross-sectional and longitudinal studies, bone formation and resorption markers, mineral metabolism and calcium-regulating factors were measured at 10, 25, 30 and 36 weeks of pregnancy and at 4 days and 1 month postpartum in women with singleton and twin pregnancies.Urinary levels of cross-linked type I collagen N-telopeptides and C-terminal telopeptides of type I collagen in women with twin pregnancy were significantly higher than those in women with singleton pregnancy and those high levels were observed earlier than those in women with singleton pregnancy. In the cross-sectional study, serum levels of bone-specific alkaline phosphatase, calcium and phosphate in women with twin pregnancy were higher and the levels of 1,25-(OH)2 vitamin D and 25-(OH) vitamin D in women with twin pregnancy were lower than those in women with singleton pregnancy.Changes in bone metabolism in women with twin pregnancy are different from those in women with singleton pregnancy. Early and large increases in bone turnover markers allow women with twin pregnancy to meet high fetal demand for calcium during pregnancy.