A detailed assessment of alterations in bone turnover, calcium homeostasis, and bone density in normal pregnancy

The effects of pregnancy on bone turnover and the potential risk of develop ing an osteoporotic fracture in pregnancy are controversial. Utilizing bioc hemical markers of bone formation and resorption and dual-energy X-ray abso rptiometry (DEXA), bone turnover before, during, and after pregnancy was st udied in detail. Ten women (mean age 30 years; range 23-40) were recruited. Prepregnancy data were obtained and then a review was performed at 2-week intervals, once pregnancy was confirmed, until 14 weeks of gestation and th ereafter monthly until term. Bone mineral density (BMD) was estimated by DE XA scanning of hip, spine, and forearm preconception and postpartum. In add ition, BMD of the forearm at 14 weeks and 28 weeks gestation was obtained. All pregnancies had a successful outcome. Urinary free pyridinium cross-lin ks, free pyridinoline (fPyr) and free deoxypyridinoline (fDPyr), were norma l prepregnancy (mean [+/-SD]) 14.6 nmol/mmol (1.8) and 5.0 nmol/mmol (1.0) creat, respectively. By 14 weeks, they had increased to 20.8 nmol/mmol (4.3 ) and 6.1 nmol mmol (1.4) (both p < 0.02) and by 28 weeks to 26.3 nmol/mmol (5.6) and 7.4 nmol/mmol (1.6) (both p < 0.01). The ratio of fPyr to fDPyr remained constant. A similar significant increase was observed in N-telopep tide (NTx). Bone formation was assessed by measurement of carboxyterminal p ropeptide of type 1 collagen (P1CP) and bone-specific alkaline phosphatase (BSAP). Neither were altered significantly before 28 weeks, but subsequentl y mean P1CP increased from 110 mu g/liter (23) to 235 mu g/liter (84) at 38 weeks and mean BSAP increased from 11.1 U/liter (5.0) to 28.6 Uniter (11.1 ) (p < 0.01 for both variables). Lumbar spine (L1-L4) BMD decreased from a prepregnancy mean of 1.075 g/cm (0.115) to 1.054 g/cm(2) (0.150) postpartum (p < 0.05). Total hip BMD decreased from a prepregnancy mean of 0.976 g/cm (2) (0.089) to 0.941 g/cm(2) (0.097) (p < 0.05). Forearm BMD at midradius, one-third distal and ultradistal decreased but did not reach statistical si gnificance. As assessed by these bone markers, in the first 2 trimesters of pregnancy, bone remodeling is uncoupled with a marked increase in bone res orption. A corresponding increase in formation markers is not observed unti l the third trimester. Spinal BMD exhibits a significant decrease from prep regnancy to the immediate postpartum period with a mean reduction in BMD of 3.5% in 9 months.