Table 4.

Associations between average trihalomethane (TTHM) exposure (second trimester) for preterm delivery, and very preterm delivery expressed as odds ratios (ORs) and 95% confidence interval (CIs).

Treatment	Components	Total births ($n$)	No chlorine (OR)	$<5\mathrm{\mu }\mathrm{g}\text{\hspace{0.17em}TTHM}/\mathrm{L}$ [OR (95% CI)]	$5–15\mathrm{\mu }\mathrm{g}\text{\hspace{0.17em}TTHM}/\mathrm{L}$ [OR (95% CI)]	$>15\mathrm{\mu }\mathrm{g}\text{\hspace{0.17em}TTHM}/\mathrm{L}$ [OR (95% CI)]	$p_{\text{trend}}$a
Preterm delivery
All chlorination treatmentsb	Cases ($n$)	548,619	3,394	12,151	6,455	3,874
	Noncases ($n$)		62,614	250,026	132,217	77,888
	Crude		1.00 (ref)	0.90 (0.86, 0.93)	0.90 (0.86, 0.94)	0.92 (0.87, 0.96)	0.8
	Model 1		1.00 (ref)	0.91 (0.87, 0.96)	0.93 (0.88, 0.98)	0.93 (0.88, 0.99)	0.09
	Model 2		1.00 (ref)	0.92 (0.88, 0.97)	0.94 (0.89, 1.00)	0.94 (0.88, 1.00)	0.1
Chloramine	Cases ($n$)	285,586	3,394	7,810	670	1,684
	Noncases ($n$)		62,614	165,202	14,570	34,845
	Model 1		1.00 (ref)	0.90 (0.85, 0.94)	0.84 (0.76, 0.93)	0.91 (0.85, 0.99)	0.1
	Model 2		1.00 (ref)	0.90 (0.85, 0.95)	0.85 (0.77, 0.94)	0.90 (0.83, 0.98)	0.06
	Model 2			1.00 (ref)	0.93 (0.85, 1.03)	1.00 (0.93, 1.07)	1.0
Hypochlorite	Cases ($n$)	166,172	3,394	1,474	1,477	2,091
	Noncases ($n$)		62,614	26,329	27,291	41,502
	Model 1		1.00 (ref)	1.01 (0.94, 1.09)	1.01 (0.94, 1.09)	0.93 (0.88, 1.00)	0.05
	Model 2		1.00 (ref)	1.04 (0.97, 1.13)	1.05 (0.97, 1.13)	0.95 (0.89, 1.02)	0.2
	Model 2			1.00 (ref)	1.04 (0.97, 1.13)	0.95 (0.88, 1.02)	0.2
Very preterm delivery
All chlorination treatmentsb	Cases ($n$)	548,619	530	1,908	1,028	580
	Noncases ($n$)		65,478	260,269	137,644	81,182
	Crude		1.00 (ref)	0.91 (0.82, 1.00)	0.92 (0.83, 1.03)	0.88 (0.78, 1.00)	0.4
	Model 1		1.00 (ref)	0.88 (0.77, 1.00)	0.91 (0.79, 1.04)	0.83 (0.71, 0.98)	0.2
	Model 2		1.00 (ref)	0.92 (0.80, 1.06)	0.94 (0.81, 1.09)	0.82 (0.69, 0.98)	0.05
Chloramine	Cases ($n$)	315,749	530	1,390	139	249
	Noncases ($n$)		65,478	194,248	17,435	36,280
	Model 1		1.00 (ref)	0.86 (0.75, 0.99)	1.04 (0.83, 1.32)	0.81 (0.67, 0.99)	0.2
	Model 2		1.00 (ref)	0.91 (0.78, 1.06)	1.13 (0.89, 1.45)	0.77 (0.62, 0.96)	0.05
	Model 2			1.00 (ref)	1.23 (0.99, 1.52)	0.85 (0.70, 1.02)	0.06
Hypochlorite	Cases ($n$)	166,172	530	225	217	315
	Noncases ($n$)		65,478	27,578	28,551	43,278
	Model 1		1.00 (ref)	0.87 (0.71, 1.07)	0.83 (0.67, 1.02)	0.84 (0.70, 1.01)	0.07
	Model 2		1.00 (ref)	0.95 (0.76, 1.17)	0.89 (0.71, 1.10)	0.88 (0.73, 1.07)	0.2

Note: Model 1 was adjusted for maternal age, BMI, household income, attained education, smoking at week 30, and country of birth by inverse probability weighting. Model 2 was as Model 1 additionally adjusted for previous miscarriages, parity, sick leave/early retirement, use of teratogenic drugs, diabetes, preeclampsia, hypertension, weight gain, and year of birth. BMI, body mass index; ref, reference; TTHM, total trihalomethanes.

^a$p_{\text{trend}}$: Linear trends across categories were tested using the median TTHM concentration within categories as a continuous variable.

^bChloramine, hypochlorite, chlorine gas, chlorine dioxide in single or combined treatment.