Relationship between bone mineral density changes in pregnancy and maternal and pregnancy characteristics: a longitudinal study

Objective. The study aims to verify whether a progressive fall in bone mineral density (BMD) values can be demonstrated using quantitative ultrasound measurements of the os calcis. The BMD change during the pregnancy was then correlated with other maternal and pregnancy characteristics to identify any high-risk factors for bone loss in pregnancy. Methods. Consecutive patients were recruited from a low-risk obstetric clinic over a period of 9 months. BMD measurements were performed at the os calcis before 18 weeks, between 28 and 32 weeks, and at 36-38 weeks using a Hologic Sahara Clinical Bone Sonometer system. A computer-derived BMD value was obtained with each measurement. Body fat composition was also measured using a Tanita 501 bioimpedance assay system. Results. In a cohort of 780 patients, a mean fall in BMD of 0.040 g/cm(2) was demonstrable across the trimesters. The difference in BMD across the three serial measurements was highly significant (p < 0.001). Univariate analysis showed that those with a low initial BMD, glucose intolerance and high body fat accumulation weight gain during the pregnancy had lower BMD loss, while gestational hypertension and obesity had no impact on the degree of BMD loss. Entering these parameters in a logistic regression analysis showed that the impact of glucose intolerance was lost, but that the other factors remained significant. A linear regression model of quantitative variables showed that only fat accumulation (p = 0.03) and early pregnancy BMD values (p < 0.001) remained significant factors associated with BMD loss. Conclusion. A gradual fall in BMD was demonstrable using ultrasound measurement of the os calcis from early to late pregnancy. Of the various parameters identified as significant factors affecting BMD loss in pregnancy, a low initial BMD in early pregnancy and high body fat accumulation during pregnancy appeared to be related to lower BMD loss.