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 greater than or equal to 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]; greater than or equal to 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.