The Effect of Pregnancy on Bone Density and Bone Turnover

Abstract

During pregnancy, the mother adapts to meet the calcium demands of the fetus. The effect of this adaptation on the maternal skeleton is not fully understood. Our objectives were to evaluate changes in bone mineral density (BMD) and bone turnover during pregnancy. We studied 16 women longitudinally, with baseline measurements before pregnancy; then at 16, 26, and 36 weeks of pregnancy; and postpartum. We measured total-body BMD and biochemical markers of bone resorption (urinary pyridinium crosslinks and telopeptides of type I collagen) and bone formation (serum bone alkaline phosphatase, propeptides of type I procollagen [PINP] and osteocalcin). We also measured parathyroid hormone (PTH), insulin-like growth factor I (IGF-I), and human placental lactogen. Postpartum, BMD increased in the arms (2.8%, P F 0.01) and legs (1.9%, P F 0.01) but decreased in the pelvis (؊3.2%, P F0.

Calcium economy in human pregnancy and lactation

“…By 2-3 months postpartum in both BF and NBF mothers, intestinal Ca absorption returns to values close to those observed pre-pregnancy or in early gestation (67,112,144,148) , although there is evidence that fractional absorption is significantly higher in BF women who have resumed menstruation compared with those who have not at the same stage postpartum (148) . Urinary Ca excretion also returns from the high levels of pregnancy to values close to those observed prepregnancy or in NPNL women (13,149) . The decrease in urinary Ca output partly reflects the reduction in glomerular filtration rate after parturition (150) .…”

“…The maternal concentrations of insulin-like growth factor-I (13,60,112) , oestrogen and human placental lactogen (104) are positively correlated with markers of bone formation and bone resorption, and insulinlike growth factor-I with net Ca balance (60) . However, no relationships have been observed during pregnancy between changes in, or absolute values of, plasma OPG, or its ratio to RANKL, and any markers of bone turnover (105,107) or BMD (105) .…”

“…As with bone resorption, this increase occurs before the third trimester, ahead of peak fetal bone accretion, and is, therefore, likely to be in anticipation of, rather than being driven by, the increased requirement for Ca (145) . The increase in 24 h urinary Ca excretion during pregnancy (3,13,67,144) is considered to be due to the combined effects of the increase in intestinal Ca absorption and the higher glomerular filtration rate associated with pregnancy and not to a change in fractional renal Ca reabsorption (3,146,147) . Fasting Ca excretion, corrected for creatinine clearance, is normal or decreased (3,103) .…”

“…Of the calciotropic hormones, PTH is reported to be either unchanged during pregnancy (67,109,144) or significantly decreased (13,39) . There is evidence to suggest that, following a nadir in early gestation, plasma PTH concentration increases during pregnancy relative to the first trimester (3,43,60) .…”

The rational use of animal models in the evaluation of novel bone regenerative therapies

“…Models that are better understood and described in the literature often have proven records of being more predictive of clinical outcomes. The age of study animals as well as gender can influence the bone repair through the action of calciotropic hormones and thus merit careful consideration [115][116][117][118][119]. For example, aged, thyroparathyroidectomized and ovariectomized (OVX) animals are known for delayed fracture healing and reduced bone mineral density; therefore, OVX animals are frequently utilized to study osteoporotic fractures because these models mimic postmenopausal women [88,120,121].…”