TABLE 3.

HRs for preterm birth (PTB) according to timing and frequency of periconceptional multivitamin use¹

			All Women		Adjusted HR (95% CI)2
	n	PTB	Crude HR	Adjusted HR (95% CI)2	BMI <25 kg/m2	BMI ≥25 kg/m2
		n (%)
Nonuser	11,503	604 (5.2)	1.00
Multivitamin user (any)	21,785	1013 (4.5)	0.88	0.89 (0.80, 0.99)	0.84 (0.73, 0.95)	1.03 (0.84, 1.26)
Preconception, postconception
−, −	11,503	604 (5.2)	1.00
++, ++	8569	376 (4.3)	0.83	0.89 (0.77, 1.02)	0.82 (0.70, 0.97)	1.05 (0.81, 1.35)
+, ++	2281	124 (5.3)	1.01	1.01 (0.82, 1.24)	0.93 (0.73, 1.20)	1.21 (0.83, 1.76)
−, ++	4297	211 (4.8)	0.93	0.92 (0.77, 1.08)	0.90 (0.74, 1.10)	0.93 (0.67, 1.28)
− or +, − or +	6638	302 (4.5)	0.87	0.84 (0.72, 0.97)	0.77 (0.65, 0.92)	1.01 (0.77, 1.32)

¹−, no use; +, 1–3 wk during the 6-wk interval; ++, 4–6 wk during the 6-wk interval.

²Cox regression models were adjusted for age, parity, BMI, sociooccupational status, and smoking. P = 0.07 for the interaction between multivitamin use and overweight status (BMI, in kg/m²: ≥25).