Quality assessment
An overview concerning risk of bias ascertained for each article is
listed in the appendix [S3 ]. Overall, biases were mostly
caused by study design (not including a healthy control group) and
patient selection as most studies included women in tertiary referral
centres. Most case series lacked describing a hypothesis or did not
discuss the biological plausibility of the hypothesis in the light of
reports’ observations [26]. Cohort studies and case control studies,
as assessed by the NOS scale [25], most frequently used hospital
controls instead of community controls, and missing data was poorly
reported.