PARITY AND BONE-MINERAL DENSITY IN MIDDLE-AGED WOMEN

A retrospective study was carried out to determine the relationship be tween parity and bone mineral density (BMD) in middle-aged women. Eigh t hundred and twenty-five woman aged 41-76 years were recruited from f our general practice registers in Cambridge. Subjects were unselected as to their health status. Each subject completed a detailed health qu estionnaire. Participation rate was 50%. The main outcome measure was BMD measured at the spine (L2-4, n=825) and hip (neck, intertrochanter and Ward's triangle; n=817) by dual-energy X-ray absorptiometry (DXA) using the Hologic QDR-1000 densitometer. It was found that the unadju sted mean BMD was significantly higher at all sites among the parous w omen (p=0.031 to <0.00001), and remained significantly higher at the f emoral neck (p=0.025), intertrochanter (p=0.001) and Ward's triangle ( p=0.045) after adjusting for age and body mass index (BMI). Similar fi ndings were seen after stratifying for potential confounding variables . There was a consistent upward trend of BMD with increasing parity at all sites. Parity remained a significant independent predictor of BMD at all sites after controlling for age, BMI, menopausal status, oral contraceptive and hormone replacement therapy use, smoking status and breast-feeding status in multiple linear regression analyses. There wa s, on average, a 1.0% increase in BMD per live birth. Our findings the refore suggest a positive relationship between parity and bone mass.