Influence of parity and lactation on hip fracture risk.

Several studies indicate that parity and lactation are associated with modest, short-term bone loss, but the long-term effect on osteoporotic fracture risk is uncertain. The authors therefore analyzed data from a population-based case-control study among Swedish postmenopausal women aged 50-81 years between October 1993 and February 1995. Mailed questionnaires and telephone interviews were used to collect data on 1,328 incident cases with hip fracture and 3,312 randomly selected controls. In age-adjusted analyses, the risk of hip fracture among all women was reduced by 10% per child (95% confidence interval (CI): 5, 14). After multivariate adjustment including body mass index as a covariate, the risk reduction was 5% per child (95% CI: 0, 10). Oral contraceptive use modified the association of parity with hip fracture risk. Among never users of oral contraceptives, the risk of hip fracture was reduced by 8% per child (95% CI: 2, 13), whereas among ever users of oral contraceptives, the risk was in the opposite direction, with an increase in risk by 19% per child (95% CI: 0, 41). After parity was considered, there was no association of duration of lactation period with fracture risk. The authors conclude that parity is modestly associated with a reduced hip fracture risk among women who had not used oral contraceptives previously.