Table 1.

Baseline population characteristics by individual maternal average exposure to the four most common trihalomethanes (TTHMs), during the second ($n=\mathrm{548,619}$) and third trimester ($n=\mathrm{576,483}$), expressed as proportions (%).

Variables	Categories	No chlorination	Second trimester			Third trimester
			$<5\mathrm{\mu }\mathrm{g}\text{\hspace{0.17em}TTHM}/\mathrm{L}$	$5–15\mathrm{\mu }\mathrm{g}$ TTHM/L	$>15\mathrm{\mu }\mathrm{g}\text{\hspace{0.17em}TTHM}/\mathrm{L}$	$<5\mathrm{\mu }\mathrm{g}\text{\hspace{0.17em}TTHM}/\mathrm{L}$	$5–15\mathrm{\mu }\mathrm{g}$ TTHM/L	$>15\mathrm{\mu }\mathrm{g}\text{\hspace{0.17em}TTHM}/\mathrm{L}$
$n$		66,008	262,177	138,672	81,762	263,687	168,449	78,439
Average	$\mathrm{\mu }\mathrm{g}\text{\hspace{0.17em}TTHM}/\mathrm{L}$	—	0.76	9.0	24	0.75	9.2	24
Outcome	SGAa	1.8	1.9	2.2	2.0	1.9	2.2	2.1
	Preterm delivery	5.1	3.8	4.1	4.0	3.8	4.1	4.0
	Very preterm delivery	0.80	0.27	0.24	0.38	0.27	0.25	0.38
Sex of child	Female	51	52	51	51	52	51	51
Year of birth	2005	8.2	8.1	7.7	7.4	8.2	7.6	7.2
	2006	8.6	8.3	8.1	7.9	8.3	8.0	7.8
	2007	8.8	8.4	8.4	8.3	8.4	8.4	8.3
	2008	8.9	8.9	8.7	8.8	8.8	8.8	8.9
	2009	9.3	8.9	9.2	9.1	8.8	9.3	9.2
	2010	9.5	9.7	9.8	9.8	9.7	9.7	9.9
	2011	9.3	9.4	9.4	9.6	9.4	9.4	9.6
	2012	9.3	9.5	9.6	9.7	9.5	9.6	9.8
	2013	9.2	9.6	9.6	9.5	9.7	9.5	9.6
	2014	9.3	9.7	9.8	9.9	9.7	9.8	9.9
	2015	9.4	9.5	9.8	10	9.5	9.8	9.8
Maternal age (y)	$<25$	17	10	11	14	10	12	14
	25–29	33	26	28	30	26	29	30
	30–34	32	38	37	35	38	37	35
	35–39	15	22	19	17	22	18	17
	$>40$	2.9	4.9	4.1	3.6	4.8	3.9	3.5
Maternal BMI ($\mathrm{kg}/{\mathrm{m}}^2$)	$<18.5$	2.4	2.8	2.7	2.5	2.9	2.6	2.6
	$18.5–<25$	59	65	63	61	65	63	61
	$25–<30$	26	22	23	25	22	24	25
	$\ge 30$	13	10	11	12	9	11	12
Prior miscarriages	Yes	20	22	21	21	22	21	21
Parity	Nulliparous	44	45	47	45	45	46	45
	1	37	37	37	37	38	37	36
	2	13	13	12	13	13	12	13
	$\ge 3$	5.6	4.7	4.7	5.9	4.7	5.0	6.1
Country of birth	Nordic	80	73	73	70	73	73	69
	Europe, other	6.3	6.4	7.1	9.1	6.3	7.0	9.3
	Africa	2.9	5.3	5.4	4.3	5.2	5.3	4.3
	North/South America	1.0	2.5	1.8	1.5	2.5	1.6	1.5
	Asia	9.4	13	13	16	13	13	16
Attained education	Elementary school	11	11	11	12	10	11	12
	Secondary education	39	32	32	31	32	32	31
	Postsecondary education	50	58	58	57	58	57	57
Household income (quartile)	First	16	16	17	21	15	17	22
	Second	29	22	25	28	22	26	27
	Third	40	30	34	33	30	35	33
	Fourth	15	33	23	18	33	22	18
Sick leave/early retirement	Yes	0.22	0.15	0.11	0.11	0.2	0.1	0.1
Smoking: gestational week 30–32 (cigarettes/d)	1–9	3.9	2.6	3.3	3.3	2.6	3.3	3.5
	$>9$	0.94	0.94	0.87	0.95	0.69	0.89	1.0
Drugs (suspected teratogens)	Yes	0.053	0.043	0.043	0.051	0.046	0.042	0.047
Maternal diagnosis	Diabetes	1.9	1.3	1.5	2.7	1.3	1.6	2.7
	Preeclampsia	2.9	2.5	2.6	2.6	2.4	2.5	2.5
	Hypertension	1.5	1.8	1.2	1.6	1.9	1.2	1.6
	High weight gain	0.014	0.012	0.014	0.097	0.012	0.040	0.12
	Low weight gain	0.015	0.0015	0.0051	0.012	0.00076	0.0065	0.013

Note: —, not applicable; BMI, body mass index; NA, not possible to evaluate the coverage; SGA, small for gestational age, excluding preterm delivery; TTHM, total trihalomethanes.

^aExcluding preterm delivery: based on $n=\mathrm{548,619}$ for second trimester exposure and $n=\mathrm{552,372}$ for third trimester exposure. There was no missing information except for BMI, sick leave, and smoking (8%, 13%, and 6% respectively).