. 2018 Jul 5;298(4):697–704. doi: 10.1007/s00404-018-4823-4

Table 1.

The characteristics of included studies

Study	Study type	Country	Study period	The period of folic acid use	Folic acid supplement	No.		Quality assessment
Study	Study type	Country	Study period	The period of folic acid use	Folic acid supplement	Suppl.	No suppl.	Quality assessment
Bodnar et al. [21]	PCS	USA	1997–2001	In the past 6 months, daily use	Periconceptional multivitamin use	860	975	7
Bukowski et al. [22]	PCS	USA	1999–2002	In the past 1 year, daily use	Preconceptional folate supplementation	12,444	15,259	7
Catov et al. [23]	PCS	Denmark	1997–2003	4 weeks before the LMP through 8 weeks after the LMP	Periconceptional multivitamin use	21,785	11,503	6
Catov et al. [23]	PCS	Denmark	1997–2003	4 weeks before the LMP through 8 weeks after the LMP	Folate-only users	2609	11,503
Charles et al. [24]	RCT	UK	1966–1967	< 37 weeks gestation	Periconceptional folic acid use	1902	917
Hernandez-Diaz et al. [25]	RCS	USA, Canada	1993–2000	From 2 months before conception through the entire pregnancy	Periconceptional folic acid use	1953	147	7
Kim et al. [26]	RCS	South Korea	2009–2010	NR	Prenatal intake of folic acid	134	81	8
Li et al. [16, 31]	RCS	China	1993–1996	During early pregnancy	Folic acid supplementation	92,731	100,823	9
Liu et al. [27]	PCS	China	2010–2012	Pre- and post-conception	Folic acid intake	7864	2315	7
Martinussen et al. [28]	PCS	USA	1996–2000	First trimester overall	Folic acid supplementation in early pregnancy	3301	346	8
Timmermans et al. [29]	PCS	Netherlands	2002–2006	Before 8 weeks	Preconceptional folate supplementation	2362	1770	9
Vanderlelie et al. [13]	PCS	Australia	2006–2011	The first trimester of pregnancy	First trimester multivitamin/mineral use	719	1066	7
Vanderlelie et al. [13]	PCS	Australia	2006–2011	The first trimester of pregnancy	Folate-only users	476	1066
Wang et al. [15]	PCS	China	2010–2012	Before conception and/or during pregnancy	Dietary folate intake before conception and during pregnancy	794	265	7
Wen et al. [30]	PCS	Canada	2002–2005	In early second trimester	Prenatal multivitamin use	2317	238	6
Wen et al. [30]	PCS	Canada	2002–2005	In early second trimester	Folate-only users	421	238
Wen et al. [14]	PCS	Canada	2002–2008	In early second trimester	Prenatal multivitamin use	7265	404	6
Wen et al. [14]	PCS	Canada	2002–2008	In early second trimester	Folate-only users	625	404

PCS prospective cohort study; RCS retrospective cohort study; RCT randomized controlled trail; Suppl supplementation; LMP last menstrual period