Changes in Bone Mineral Status and Bone Size During Pregnancy and the Influences of Body Weight and Calcium Intake

Abstract

ABSTRACT

Additional calcium is required during pregnancy for fetal skeletal mineralization. The extent of the contribution of maternal bone mineral to calcium economy in human pregnancy is unclear. It is possible that the calcium intake of the mother before or during pregnancy and differences in maternal body weight and weight gain may influence the magnitude of the skeletal response in pregnancy.

This prospective study examined changes in whole body and regional bone mineral status and in bone size before and after delivery and the influence of body weight and calcium intake during pregnancy, compared with those in nonpregnant nonlactating (NPNL) women. The study group was 34 healthy 23- to 37-year-old pregnant British women who subsequently delivered a healthy singleton baby. The control group was 84 NPNL women studied over the same time period. Participants were evaluated before they became pregnant and at 2 weeks postpartum. Dual-energy x-ray absorptiometry was used to determine pregnancy-related changes in bone mineral content, bone area, areal bone mineral density, and bone area-adjusted bone mineral content of the whole-body, lumbar spine, radius, and hip. Daily calcium intake was estimated by measuring dietary calcium and the intake of calcium from supplements.

In the study group, significant decreases of 1% to 4% were found in bone mineral content, areal bone mineral density, and bone area-adjusted bone mineral content at the whole body, spine, and total hip. Among the women in the NPNL group, areal bone mineral density and bone area-adjusted bone mineral content decreased at the spine and hip by only 0.5% to 1%, and unlike the study group, significant increases were found in whole-body bone mineral content. The mean decrease in whole-body bone mineral content among women in the study group was −2.16% ± 0.46%, an estimate relative to the change in the NPNL group [−2.78% ± 0.49%, (P ≤ 0.001)]. Between both groups, change in weight was a positive predictor of skeletal changes at the hip, spine, and radial shaft; either bone mineral status was greater or the decrease was less in those who gained weight. After correction for weight change and other variables, significant differences between the study and NPNL groups were found in the change in bone area-adjusted bone mineral content as follows (P ≤ 0.01): whole-body, −1.72% ± 0.29%; spine, −3.03% ± 0.72%; and total hip, −1.87% ± 0.60%. Calcium intake was not a significant predictor of change in bone mineral status or bone size at any site in either group.

These findings provide evidence that skeletal changes in well nourished pregnant women are independent of calcium intake. The investigators suggest that changes in body weight should be considered when estimating the magnitude of the skeletal response to pregnancy.