Table 3.

Relationship between maternal plasma folate levels and preterm birth (PTB) among predominantly urban, low-income, minority mother–infant dyads (n 2313) enrolled in the Boston Birth Cohort, USA, from 1998 to 2014

Plasma folate level	Term (n 1593)		PTB (n 720)		Unadjusted		Adjusted OR*
	ncases	%	ncases	%	OR	95 % CI	OR	95 % CI
Continuous plasma folate concentration (nmol/l)
Each unit increase†	n/a	n/a	n/a	n/a	0·991	0·986, 0·995	0·994	0·990, 0·999
Each interquartile increase in folate level‡	n/a	n/a	n/a	n/a	0·81	0·73, 0·90	0·88	0·79, 0·97
Quartiles of plasma folate concentration (nmol/l)
Lowest quartile: 6·6–19·4 (ref.)	378	23·7	201	27·9	1·00	n/a	1·00	n/a
Second quartile: 19·4–30·0	386	24·2	192	26·7	0·94	0·73, 1·19	1·02	0·79, 1·33
Third quartile: 30·0–43·8	393	24·7	185	25·7	0·89	0·69, 1·13	1·09	0·83, 1·42
Highest quartile: 43·8–185·5	436	27·4	142	19·7	0·61	0·47, 0·79	0·74	0·56, 0·97
WHO classification (nmol/l)
Insufficiency/deficiency: <13·5	155	9·7	82	11·4	1·07	0·80, 1·43	0·86	0·63, 1·18
Normal: 13·5–45·3 (ref.)	1030	64·7	508	70·6	1·00	n/a	1·00	n/a
Excess: >45·3	408	25·6	130	18·1	0·65	0·52, 0·81	0·70	0·55, 0·89

ref., reference category; n/a, not applicable.

^*Adjusted for maternal race, age, nativity, education, marital status, receipt of public assistance, parity, tobacco use, alcohol use, stress, BMI, pre-eclampsia disorders, intra-uterine infection/inflammation, and diabetes.

^†Mean (sd) for term birth and PTB: 36·6 (24·7) and 32·3 (20·0) nmol/l, respectively.

^‡Median (interquartile range) for term birth and PTB: 31·5 (20·8–45·6) and 28·5 (19·0–38·5) nmol/l, respectively.