Table 2.

Risk of bias assessment

Study	Selection					Comparability		Outcome			Stars
Author	Publication year	Representativeness of the exposed cohort	Selection of the non-exposed cohort	Ascertainment of exposure	Demonstration that outcome of interest was not present at start of study	Study controls for age	Study controls for multiple pregnancies	Assessment of outcome	Long enough follow-up	<10 % lost to follow-up
Klasky	2010	*	*	*	*	*	*	*	*	*	9
Krieg	2008	*	*	*	*			*	*	*	7
Le Ray	2012	*	*	*	*			*	*	*	7
Levron	2014	*	*	*	*		*	*	*	*	8
Malchau	2013	*	*	*	*		*	*	*	*	8
Sekhon	2014	*	*	*	*	*	*	*	*	*	9
Söderström-Anttila	1998	*	*	*	*	*		*	*	*	8
Van Dorp	2014	*	*	*	*	*	*	*	*	*	9
Stoop	2012	*	*	*	*	*	*	*	*	*	9
Tranquilli	2013	*	*	*	*			*	*	*	7
Wiggins	2005	*	*	*	*		*	*	*	*	8

* indicates a star, cells with a star indicate that the corresponding item is addressed satisfactorily in the publication in question