Table 3.

Quality assessment according to the Newcastle–Ottawa scale

Study	Selection	Comparability	Exposure	Total score
Cooper (2018) [15]	3	2	3	8
Dingemans (2018) [17]	3	2	2	7
Reddix (2010) [10]	3	1	3	7
Zhou (2016) [18]	3	2	2	7