Risk of bias-assessment
Among the eligible studies, 7 (44%) were of high quality. Most studies
were at low risk for selection bias (12/16, 75%) and all studies
controlled for at least one confounding factor (maternal age, body mass
index (BMI) and parity). Given we only considered studies using
validated questionnaires for patient self-reporting, by definition we
scored all studies at risk of bias for the assessment of outcome. Nine
of the included studies for quantitative synthesis (9/16, 56%) were at
high risk of bias for outcome/exposure domain, though. Details on
quality assessment and risk of bias for individual studies are displayed
in Appendix 2B.