Table 2

Risk of bias assessment in included studies

EPOC criteria for randomised controlled trials and controlled before-and-after studies
Study No	Reference	Adequate sequence generation	Adequate concealment of allocation	Similar baseline outcome measures	Similar baseline characteristics	Adequate follow-up	Blinding of outcome measures	Adequate protection against contamination	Free from selective outcome reporting	Free from other risks of bias	–
1	Chambliss et al23	?	+	?	+	−	+	+	+	+
2	Law and Lam24	+	+	?	+	+	+	+	+	+
3	Bernitz et al25,26	+	+	?	+	+	+	+	+	+
4	Hofmeyr et al27	−	−	?	?	+	+	+	+	+
Newcastle-Ottawa Scale and STROBE quality assessment tool for cohort studies
Study No	Reference	Representativeness*	Selection of the controls*	Ascertainment of exposure*	Outcomes not present at the start*	Comparability for parity*	Assessment of outcome*	Follow-up duration*	Adequate follow-up*	All outcome measures reported†	Authors discuss sources of bias†
5	Homer et al28	+	+	+	+	−	+	+	+	+	−
6	Rana et al29	+	+	+	+	−	+	+	+	+	+
7	Eide et al30‡	+	+	+	?	NA	?	+	+	+	−
8	Suzuki et al31, Suzuki32	+	+	+	+	+	+	+	+	+	−
9	Cheung et al33 34	+	+	+	−	−	−	+	+	+	+
10	Brocklehurst et al35; Schroeder et al36	+	+	+	+	+	+	+	+	+	+

+, low risk of bias; −, high risk of bias; ?, unclear risk of bias.
*Newcastle-Ottawa Scale.
†STROBE quality assessment tool.
‡Eide et al30 only recruited nulliparous women.
EPOC, Effective Practice and Organisation of Care group; NA, not applicable; STROBE, Strengthening the Reporting of Observational Studies in Epidemiology.