Table 2.

The risk of bias of the included studies using Cochrane Collaboration criteria for quality assessment.

Item/Author, year	Chang, 2013	Gupta, 2019	Kalk, 2020	Moro, 2018	Salazar-Austin, 2019	Taylor, 2013	Figueroa-Damia, 1998	Sharma, 2016	Loveday, 2020	Czeizel, 2001
Random sequence generation (selection bias)	H	L	H	L	H	L	H	L	H	H
Allocation concealment (selection bias)	U	L	H	L	H	L	H	L	H	H
Blinding of participants and personnel (performance bias)	H	L	H	L	H	H	H	L	U	L
Blinding of outcome assessment (detection bias)	H	L	U	U	U	H	L	L	H	H
Incomplete outcome data (attrition bias)	L	L	L	L	H	L	L	L	L	L
Selective reporting (reporting bias)	H	U	L	H	H	U	U	L	U	U
Other bias	H	L	H	L	U	L	H	U	H	H

H: high; L: low; U: unclear.