Published Online
on December 4, 2003

A more recent version of this article appeared on January 1, 2004

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Submitted on September 3, 2003
Accepted on September 5, 2003

Parity and Risk of Subarachnoid Hemorrhage in Women. A Nested Case-Control Study Based on National Swedish Registries

David Gaist MD, PhD^*; Lars Pedersen MSc; Sven Cnattingius MD, PhD; and Henrik Toft Sørensen MD, PhD

From the Department of Clinical Epidemiology, Aarhus and Aalborg University Hospital, Aarhus, Denmark (D.G., L.P., H.T.S.); Department of Epidemiology, Institute of Public Health, University of Southern Denmark, Odense, Denmark (D.G.); Department of Neurology, Odense University Hospital, Odense, Denmark (D.G.); and Department of Medical Epidemiology, Karolinska Institute, Stockholm, Sweden (S.C.).

^* To whom correspondence should be addressed. E-mail: dg{at}dadlnet.dk.

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 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]; 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.

Key words: cigarette smoking • epidemiology • parity • stroke • subarachnoid hemorrhage

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