Table 2.

Baseline characteristics of individual studies on blood folate concentration and infant asthma.

Author	Year	Region	N	RR (95%CI)	Sample type	Adjustment for covariates
den Dekker H. T. et al.29	2018	Netherlands	276	0.93 (0.79–1.09)	Serum	Maternal age and BMI at intake, parity, history of asthma or atopy, educational level, smoking or alcohol use during pregnancy, child’s gestational age at birth, birthweight and ethnicity
Parr C. L. et al.30	2017	Norway	2,681	0.97 (0.54–1.76)	Plasma	Maternal age at delivery, parity maternal education, prepregnancy BMI, maternal history of atopy, maternal smoking in pregnancy, use of cod liver oil and other dietary supplements in pregnancy, and gestational week of sample collection
Magdelijns F. J. et al.35	2011	Netherlands	2640	0.31 (0.09–1.10)	In erythrocytes	Maternal antibiotic use during pregnancy, maternal smoking during pregnancy, maternal alcohol consumption during pregnancy, mode and place of delivery, birth weight, gender of the child, treatment with antibiotics during the first 6 months of life, breastfeeding duration, exposure to domestic animals during pregnancy and the first 2 years of life, exposure to environmental tobacco smoke in the first 6 to 7 years of life, siblings, family history of atopy, recruitment group, maternal education level, day care, and multivitamin or other supplement use during pregnancy.
van der Valk R. J. et al.37	2013	Netherlands	2,001	1.02 (0.83–1.25)	Cord blood	Maternal age, BMI, educational level at intake, history of maternal atopy or asthma, smoking and folic acid supplement use during pregnancy, parity and children’s sex, gestational age and birth weight
Håberg S. E. et al.38	2011	Norway	1962	1.66 (1.16–2.37)	Plasma	Maternal educational level, maternal age, parity, maternal atopy, maternal BMI, maternal smoking in pregnancy and maternal smoking at age 3 years, supplement use at age 3 years