Finding decreased radius and spine bone mineral density during the 6 months of lactation might prompt us to consider the risk for osteoporosis, which might lead to fractures in lactating women.13,14 However, bone mineral density values at 18 months were higher than those in early postpartum in our study, whereas others found recovery of bone mineral density even in women who become pregnant during lactation.15 Our results regarding changes in bone density of the lumbar spine over the first 12 months postpartum agree with those of other authors.2–5
Oral calcium supplements yielded significant variations in comparison with women who did not take calcium; however, the bone mineral density data at 18 months postpartum suggested that calcium supplementation might have only a transient effect on postpartum bone mineral changes, resulting in a negligible effect on bone strength. Similar findings were reported by Prentice15 and Kalkwarf et al.16 Our study was limited by our not taking into account dietary calcium intake. Additional bias might have been introduced by our providing supplemental calcium openly.
We found that bone mineral density variations of the radius were related to those of the spine, a relationship also reported by Wardlaw and Pike,13 but negated by others.2,3,7 Our finding might be explained by the fact that measurements are done automatically by the scanning system where the trabecular component in the radius was comparable to that of the spine.10 Mineral loss during lactation was found mostly at the level of trabecular bone.
In nonlactating women, the relative increment of bone mineral density over the 18-month period might be accounted for by the early return of ovarian function. Differing patterns of bone loss were found among lactating women according to whether they resumed menstruation within 3–6 months postpartum, in accordance with findings reported by Sowers et al14 and Kalkwarf and Specker.3 Without regular ovarian function, the amount of bone loss almost doubles that found in women with regular menses. After the first 6 months postpartum, women who lost more bone mass showed a more pronounced increase in bone mineral density compared with those who lost less. At 18 months postpartum, bone mineral density values were comparable between those with early and late return of menstruation. Serum estradiol or prolactin were not measured in our study, so we could not characterize the relationship between hormonal patterns and bone mass.
Unlike most studies of lactation, our protocol provided all women with the fixed time limits of 6 months for lactation and 1 month for weaning. Cabergoline administration at 7 months postpartum to women who breast-fed decreased prolactin levels and restored the function of the pituitary-ovarian axis more rapidly. In theory, that might have been beneficial to bone mass recovery.
Received August 31, 1998. Received in revised form November 30, 1998. Accepted December 30, 1998.