Cohort or case-control studies that controlled for age and that evaluated progestogen-only contraceptives (POC) with clearly defined contraceptive users and never and past users, and that reported on the incidence of any type of stroke (ischaemic and haemorrhagic) were eligible for inclusion. Studies of hormone replacement therapy were excluded.
The included studies were all case-control studies conducted between 1989 and 1998 of women aged between 15 and 44. Most studies evaluated oral POC; one was oral and injectable and one was of Norplant. POC users were defined as taking POC at the time of the event or within one to three months before. Controls were either population, hospital or both. All studies adjusted for age in addition to one or more of smoking, education, alcohol consumption, migraine, high blood pressure and marital status. Studies defined stroke as any type or ischaemic only.
Studies were selected by two reviewers.