Effect of bed rest during pregnancy on bone turnover markers in pregnant and postpartum women.

OBJECTIVE: The aims of our study were to evaluate the changes in bone turnover markers during pregnancy and puerperium as a longitudinal study and to elucidate the effect of bed rest during pregnancy on bone turnover markers in pregnant and postpartum women.

METHODS: The study population comprised 27 Japanese pregnant women aged 23-40 years. All women were recruited for the longitudinal study from the outpatients clinic of the Department of Obstetrics and Gynecology, Tokushima University Hospital. Concentrations of serum bone-specific alkaline phosphatase (BAP), urinary cross-linked type I collagen N-telopeptides (NTx), serum NTx and urinary C-terminal telopeptide of type I collagen (CTx) were measured at 10, 26, 30 and 36 weeks of pregnancy and at 4 days and 1 month postpartum. In addition, we recruited 15 pregnant women (aged 25-35 years) who were treated by bed rest before 30 weeks of pregnancy for threatened premature delivery and compared bone turnover markers in these women with those in 22 normal pregnant women (aged 22-39 years). Concentrations of serum BAP, serum NTx, urinary NTx and urinary CTx were measured at 30 and 34 weeks of pregnancy and at 4 days and 1 month postpartum.

RESULTS: In the longitudinal study, serum BAP concentration at 1 month postpartum was significantly higher than that at any stage of pregnancy and that at 4 days postpartum. Urinary concentration of NTx increased gradually during pregnancy and showed a peak at 36 weeks of pregnancy, followed by a decrease in the postpartum period. Serum NTx concentration and urinary CTx concentration showed the same patterns of change as that of urinary NTx concentration. In the comparison study, urinary concentrations of NTx and CTx at 30 and 34 weeks of pregnancy in women with bed rest were significantly (p<0.0001 and p<0.001, respectively) higher than those in normal pregnant women. Serum NTx concentration at 34 weeks of pregnancy in women with bed rest was also significantly (p=0.0029) higher than that in normal pregnant women. Serum BAP concentration at 34 weeks of pregnancy in women with bed rest was significantly (p=0.0038) higher than that in normal pregnant women, and these high levels were maintained during puerperium. Serum BAP concentration at 34 weeks of pregnancy was significantly correlated with duration of bed rest (r=0.767, p=0.0041).

CONCLUSION: Immobilization due to bed rest during pregnancy is associated with increases in bone turnover markers in pregnant and postpartum women. Concentrations of bone resorption markers increased rapidly at the start of bed rest, while the concentration of a bone formation marker gradually increased toward puerperium.