Table 3. Methodological quality of the included studies acoording to the Newcastle-Ottawa Scale.

Study	Selection	Comparability	Exposure	Total
Roberts et al. 21	**	*	**	5
Shelton et al. 22	****	*	**	7
Von Ehrenstein et al. 23	**	**	**	6
Cheslack-Postava et al. 13	**	**	**	6
Lyall et al. 20	***	**	***	8
Brown et al. 19	***	*	**	6

Selection: Maximum 4 points awarded for the case definition, representativeness of the cases, selection of controls, and definition of controls. Comparability: Maximum 2 points awarded for controlling for the pre-specified primary confounding variable (age) and additional confounding variables. Exposure: Maximum 3 points ascertainment of exposure, method of ascertainment for cases and controls, and non-response rate. Total: A maximum of 9 points could be awarded.