Pregnancy and lactation as determinants of bone mineral density in postmenopausal women.

The relation of pregnancy and breast feeding to bone mineral density of the wrist, radius, hip, and spine was examined in a white, upper middle-class, homogeneous sample of 741 postmenopausal women ranging in age from 60 to 89 years. Number of pregnancies ranged from 0 to 14, with a mean of 2.0 pregnancies and 1.5 live births. Almost two thirds of the women who had had a live birth reported breast feeding. Unadjusted comparisons indicated that bone mineral density of the wrist, radius, and hip increased with increasing numbers of pregnancies, and women who had breast-fed had higher bone mineral densities at these sites. However, after adjustment for age or age and body mass index, these associations were no longer significant. Multiple regression analyses adjusted for age, age at menopause, obesity, cigarette smoking, and estrogen and thiazide use also indicated that number of pregnancies and breast feeding were not significantly associated with bone mineral density at any of the four sites measured. Results of the present study suggest that reproductive history and breast feeding are not long-term determinants of bone mineral density.