Additional 1: Table S2. Initiation of pre-conceptional folic acid supplementation and risk of spontaneous preterm delivery (sPTD), depending on dietary folate intake.

Initiation of pre-conceptional folic acid supplementation (Q1 data) and hazard ratios for spontaneous PTD (n=1,628 for 22+0-36+6 weeks, n=264 for early (22+0-33+6 week), n=1,364 for late (34+0-36+6 weeks)) stratified for dietary folate intake. Cox regression for 65,668 participants in the Norwegian Mother and Child Cohort Study (2002 – 2009). Iatrogenic deliveries have been censored in the regression model.

sPTD / Dietary / Initiation of folic acid / unadjusted / adjusted1 / adjusted2
folate (µg/d) / supplementation / n / HR / (CI) / p / HR / (CI) / p / HR / (CI) / p
<170 / No / 574 / 1 / 1 / 1
0-8 w preconceptional / 179 / 1.00 / (0.84; 1.18) / 0.96 / 1.07 / (0.90; 1.27) / 0.43 / 1.09 / (0.91; 1.31) / 0.33
all / >8 w preconceptional / 260 / 1.17 / (1.01; 1.35) / 0.04 / 1.21 / (1.03; 1.41) / 0.02 / 1.22 / (1.04; 1.45) / 0.02
≥170 / No / 354 / 1 / 1 / 1
0-8 w preconceptional / 102 / 0.85 / (0.68; 1.07) / 0.16 / 0.91 / (0.73; 1.14) / 0.43 / 0.91 / (0.72; 1.15) / 0.42
>8 w preconceptional / 168 / 1.11 / (0.92; 1.33) / 0.27 / 1.16 / (0.95; 1.40) / 0.14 / 1.14 / (0.92; 1.41) / 0.23
<170 / No / 94 / 1 / 1 / 1
0-8 w preconceptional / 33 / 1.12 / (0.75; 1.67) / 0.57 / 1.30 / (0.87; 1.96) / 0.2 / 1.29 / (0.84; 1.97) / 0.24
early / >8 w preconceptional / 50 / 1.37 / (0.97; 1.93) / 0.07 / 1.54 / (1.08; 2.22) / 0.02 / 1.53 / (1.03; 2.27) / 0.03
≥170 / No / 45 / 1 / 1 / 1
0-8 w preconceptional / 13 / 0.84 / (0.45; 1.55) / 0.57 / 0.89 / (0.47; 1.66) / 0.70 / 0.8 / (0.42; 1.53) / 0.51
>8 w preconceptional / 29 / 1.46 / (0.92; 2.34) / 0.11 / 1.50 / (0.92; 2.46) / 0.10 / 1.31 / (0.76; 2.24) / 0.33
<170 / No / 480 / 1 / 1 / 1
0-8 w preconceptional / 146 / 0.97 / (0.81; 1.17) / 0.76 / 1.03 / (0.85; 1.24) / 0.77 / 1.05 / (0.87; 1.28) / 0.60
late / >8 w preconceptional / 210 / 1.13 / (0.96; 1.33) / 0.14 / 1.14 / (0.97; 1.36) / 0.12 / 1.17 / (0.97; 1.40) / 0.10
≥170 / No / 300 / 1 / 1 / 1
0-8 w preconceptional / 89 / 0.86 / (0.68; 1.09) / 0.20 / 0.92 / (0.72; 1.17) / 0.49 / 0.93 / (0.72; 1.19) / 0.55
>8 w preconceptional / 139 / 1.06 / (0.86; 1.29) / 0.60 / 1.10 / (0.89; 1.36) / 0.36 / 1.11 / (0.88; 1.40) / 0.37

1 Cox regression, adjusted for maternal age, prepregnancy BMI, parity, history of PTD and spontaneous abortion, child’s sex, smoking habits and alcohol consumption during pregnancy, maternal education, marital status, household income, energy intake and dietary folate intake.
2 Adjustment as above as well as for first trimester folic acid supplementation and pre-conceptional and first trimester vitamin A supplementation.