Table 2.

Quality assessment of included studies

First author	Year	Selection	Comparability	Exposure	Total
Kang16	2018	4	1	2	7
Luna17	2017	4	2	2	8
Strati18	2017	4	1	2	7
Kang19	2017	4	1	2	7
Inoue20	2016	4	2	2	8
Son21	2015	4	1	2	7
Kang22	2013	4	1	2	7
De Angelis23	2013	4	2	2	8
De Angelis23	2013	4	2	2	8
William24,25	2012	4	2	2	8
Gondalia26	2012	4	1	2	7
Finegold27	2010	4	1	2	7
Tomova28	2015	4	1	2	7
Wang29,30	2011	4	1	2	7
Song31	2004	3	1	2	6
Helena32	2005	3	1	2	6