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(2)) in women aged > or = 18 y. Analyses were performed on 3 data sets: study 1, 83 female twin pairs (21 monozygous and 62 dizygous) aged (x +/- SD) 42.2 +/- 15.5 y who were discordant for ever having been pregnant beyond 20 wk; study 2, 498 twin pairs aged 42.3 +/- 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 > or = 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.

Hormone replacement therapy and bone mineral density: a co-twin approach.

OBJECTIVE: Our objective was to estimate the difference in bone mass at clinically relevant sites within female twin pairs who were discordant for use of hormone replacement therapy (HRT). METHODS: We studied 46 female twin pairs who were discordant for HRT use. Bone mineral content and density were measured at the lumbar spine, total hip, femoral neck, 13 total forearm, and the total body. HRT use, calcium intake, physical activity, alcohol intake, and lifetime smoking were determined by questionnaire. RESULTS: Within a pair, lumbar spine bone mineral density was significantly greater in past and current HRT users compared with nonusers (6.2% +/- 2.0%; P = 0.006). In those pairs who were currently using HRT, the within-pair difference in lumbar spine bone density was 7.8% +/- 2.1% (P = 0.002), and a significant within-pair difference in forearm bone density (5.1 +/- 2.1%; P = 0.02) was apparent. A significant difference (4.6%; P = 0.03) was observed in total body bone mineral content when an adjustment was made for age, lean mass, fat mass, and height. CONCLUSIONS: In keeping with randomized clinical trial findings, these results indicate that HRT in routine clinical use protects significantly against menopausal bone loss at the lumbar spine and the forearm. Our results also quantify the magnitude of the benefit of HRT on bone density that might be anticipated in clinical practice.