Hip osteonecrosis and pregnancy in healthy women.

PURPOSE: Osteonecrosis has been reported to be associated with pregnancy in the absence of other known risk factors for the disease. Few population-based data exist to support an association since the association is rare. We investigated the association of the femoral head and pregnancy to define if pregnancy was a risk factor and to define the risk period.
METHODS: Using a case-crossover method design among 652 patients who were healthy (without any known cause of osteonecrosis) during or after pregnancy, we defined the periods of risk based on the timing of reported osteonecrosis and pregnancy. We compared each patient's likelihood of osteonecrosis during a ten years period including five years antepartum, pregnancy and five years postpartum with nine month intervals for the ten year and three month intervals for the two years after gestation.
RESULTS: For the 436 women who had only one child and one hip osteonecrosis, the incidence of osteonecrosis was 71.8% during pregnancy and the postpartum period, compared with 28.2% during the equivalent antepartum period for this population. Nine months after delivery, the risk of osteonecrosis declined progressively over time, from an odds ratio of 14.5 (95% confidence interval, 8.2-18.3) in last trimester of pregnancy. After the 27th month following onset of pregnancy, the relative risk was no more significantly different from the baseline risk observed in the antepartum period of women who had osteonecrosis before gestation. For women with several children, a subsequent pregnancy was not associated with osteonecrosis.
CONCLUSIONS: A risk of hip osteonecrosis is present during the end of pregnancy and after delivery, and appears to decrease quickly.