Pregnancy and lactation have no long-term deleterious effect on measures of bone mineral in healthy women: a twin study

Background: The long-term effects of pregnancy and lactation on measures of bone mineral in women remain unclear. Objective: We studied whether pregnancy or lactation has deleterious long-term effects on bone mineral in healthy women. Design: We measured bone mineral density (BMD; g/[cm.sup.2]) in women aged [greater than or equal to] 18 y. Analyses were performed on 3 data sets: study 1, 83 female twin pairs (21 monozygous and 62 dizygous) aged ([bar]x [+ or -] SD) 42.2 [+ or -] 15.5 y who were discordant for ever having been pregnant beyond 20 wk; study 2, 498 twin pairs aged 42.3 [+ or -] 15.0 y; and study 3, 1354 individual twins, their siblings, and family members. Results: In study 1, there were no significant within-pair differences in unadjusted BMD or BMD adjusted for age, height, and fat mass at the lumbar spine or total-hip or in total-body bone mineral content (BMC; kg) (paired t tests). In study 2, there was no significant within-pair difference in measures of bone mineral or body composition related to the within-pair difference in number of pregnancies. In study 3, subjects with 1 or 2 (n = 455) and [greater than or equal to] 3 pregnancies (n = 473) had higher adjusted lumbar spine BMD (2.9% and 3.8%, respectively; P = 0.001) and total-body BMC (2.2% and 3.1%; P < 0.001) than did nulliparous women (n = 426). Parous women who breast-fed had higher adjusted total-body BMC (2.6%; P = 0.005), total-hip BMD (3.2%; P = 0.04), and lower fat mass (10.9%; P = 0.01) than did parous non-breast-feeders. Conclusion: We found no long-term detrimental effect of pregnancy or breast-feeding on bone mineral measures. KEY WORDS Pregnancy, lactation, bone mineral density, twins, bone mineral content, women