Bone turnover markers during lactation, postpartum amenorrhea and resumption of menses

Changes in bone turnover, and consequent bone loss and recovery during lactation and the postweaning period, are likely modulated by varying estrogen levels inherent in these time periods. To address this question we measured serum biochemical markers of bone formation (bone-specific alkaline phosphatase, amino-terminal propeptide of type I procollagen, osteocalcin), of bone resorption (type I collagen carboxy-terminal telopeptide), and serum female sex hormones (estradiol, luteinizing hormone and follicle-stimulating hormone) in 32 healthy mothers prospectively after delivery, 3 months postpartum, after postpartum amenorrhea and 1 year after resumption of menses. During postpartum amenorrhea (mean 5.7, SD 2.9 months) bone mineral density decreased significantly, some 2% at the lumbar spine and some 3% at the femoral neck, but subsequently recovered completely at the former site and partially at the latter. Bone turnover marker levels were elevated at parturition and still at the end of postpartum amenorrhea. Subsequent to parturition the bone resorption marker level showed a decreasing trend while the formation marker levels continued increasing, and eventually coincided with the resorption level within the very first months postpartum. Both lactation and hormonal status modulated bone turnover marker levels. Maternal age was positively associated with increased bone turnover. Interestingly, higher parity and longer history of previous lactation were associated with lower bone turnover marker levels postpartum as compared with previously nulliparous women of the same age. The regression models explained typically some 20–30% of the variability in the bone turnover marker levels. The dynamic pattern in bone turnover is dissimilar to that occurring at menopause and it indicates that the bone loss most likely occurs in the beginning of postpartum period. It also seems that estrogen has a specific influence on bone turnover only during the first months of lactation.