Influence of breastfeeding and other reproductive factors on bone mass later in life.

The influence of reproductive factors on bone mass at six skeletal sites was assessed in an age-stratified random sample of white women residing in Rochester, Minnesota. After age-adjustment, whether or not women had ever breastfed, total duration of breastfeeding and duration of breastfeeding per child were not associated with reduced bone mineral, but breastfeeding for more than 8 months was associated with greater bone mineral at some sites. There were no consistent effects on bone mineral, after adjusting for age, of gravidity or parity, age at menarche, age at first delivery, use of oral contraceptives or estrogen replacement therapy, various sex hormones, nor any of the other reproductive factors assessed. There was a strong protective effect of obesity, which was also correlated with a number of the reproductive variables. While animal studies suggest that pregnancy and lactation may be associated with calcium loss from the skeleton, these data indicate that such factors have little long-term impact on bone mass in humans and little potential for identifying women at high risk of osteoporosis later in life.