Table 4.

Initiation of pre-conceptional folic acid supplementation and risk of spontaneous preterm delivery (sPTD)

sPTD	Initiation of folic acid		Unadjusted			Adjusted1			Adjusted2
	supplementation	n	HR	(CI)	p	HR	(CI)	p	HR	(CI)	p
	No	919	1			1			1
All	0-8 w preconceptional	281	0.94	(0.82–1.07)	0.36	1.01	(0.88–1.16)	0.91	1.02	(0.88–1.17)	0.82
	>8 w preconceptional	428	1.14	(1.02–1.28)	0.02	1.19	(1.05–1.34)	0.01	1.19	(1.04–1.35)	0.01
	No	139	1			1			1
Early	0-8 w preconceptional	46	1.02	(0.73–1.42)	0.92	1.15	(0.82–1.62)	0.43	1.10	(0.77–1.57)	0.59
	>8 w preconceptional	79	1.39	(1.06–1.84)	0.02	1.53	(1.14–2.04)	0.004	1.45	(1.05–1.99)	0.02
	No	780	1			1			1
Late	0-8 w preconceptional	235	0.93	(0.8–1.07)	0.30	0.98	(0.85–1.14)	0.83	1.00	(0.86–1.17)	0.98
	>8 w preconceptional	349	1.10	(0.97–1.25)	0.14	1.13	(0.99–1.29)	0.07	1.14	(0.99–1.32)	0.07

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 weeks), n = 1,364 for late (34⁺⁰-36⁺⁶ weeks)). 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.

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.