The relationship between parity and bone mineral density in women characterized by a homogeneous lifestyle and high parity.

CONTEXT: We reported previously that Old Order Amish (OOA) women have fewer hip fractures and higher bone mineral density (BMD) than non-Amish Caucasian women.
OBJECTIVE: The objective of this study was to determine whether the high parity characteristic of OOA women contributes to their relative bone health. Previous data on the long-term effects of parity on BMD have yielded conflicting results with few data from very high parity populations. This observational study included participants in the Amish Family Osteoporosis Study, begun in 1997 to identify genetic and clinical determinants of osteoporosis in the OOA. We measured BMD by dual-energy x-ray absorptiometry at the spine, hip, and distal radius in 424 parous OOA women aged 40 and older (mean age, 57.7 +/- 12 yr; mean parity, 7.6 +/- 2.9).
RESULTS: Increasing parity was associated with later menopause (P = 0.001) and modestly, but not significantly, higher body mass index (BMI) (P = 0.09). Increasing parity was associated with higher BMD at the total hip and trochanter (age-adjusted P = 0.02 and 0.03), no longer statistically significant after accounting for BMI. Among women aged 50-59 yr, parity was strongly associated with BMD even after accounting for age and BMI (age-adjusted P = 0.02), although this was not true for women younger than 50 or at least 60 yr old.
CONCLUSIONS: We conclude that high parity is associated with increased hip BMD in OOA women, largely mediated by higher BMI. The parity-hip BMD association remained statistically significant after accounting for age and BMI only in women aged 50-59 yr, partially explained by a later menopausal age with high parity. The benefit of high parity on BMD appeared to be lost soon after the menopausal transition, and, therefore, these data provide evidence of neither a detrimental nor beneficial effect of high parity on long-term bone health.