Parity and risk of subarachnoid hemorrhage in women: a nested case-control study based on national Swedish registries.

BACKGROUND AND PURPOSE Subarachnoid hemorrhage (SAH) is the only type of stroke with female predominance, suggesting that reproductive factors may play a role in the etiology. We conducted a population-based study to examine the influence of parity on the risk of SAH in women. METHODS We linked data from 3 national Swedish registries to identify first-ever hospitalizations for SAH in a cohort of women followed up since first childbirth during 1973-1997. Within this cohort, we conducted a nested case-control study and estimated the odds ratio (OR) and 95% CI of SAH by parity adjusted for age, calendar period, and length of follow-up. Information on smoking habits before the subject's first childbirth was available in a subset of the data (women with first childbirths during 1982-1997). RESULTS Of the 887 cases identified, 70% had suffered from SAH > or =5 years after giving birth to their last child. The OR declined with increasing parity (1 child: reference; 2: OR=0.83 [95% CI, 0.70 to 0.99]; 3: OR=0.72 [95% CI, 0.58 to 0.91]; 4: OR=0.72 [95% CI, 0.48 to 1.08]; > or =5: OR=0.67 [95% CI, 0.32 to 1.41]). Adjusting for daily cigarette consumption before first childbirth in the subsample in which this information was available reduced but did not eliminate the association of the disorder with parity. CONCLUSIONS Parity may confer a moderate long-term protective effect on the risk of SAH. The biological mechanism underlying this association is currently unknown.