Table 3.

Main and stratified analysis of association between pregnancy exposure to NO₂ and birth weight

	Crude			Model 1a			Model 2b			Model 3c
	N	Beta 95 % CI	p-value	N	Beta 95 % CI	p-value	N	Beta 95 % CI	p-value	N	Beta 95 % CI	p-value
Main analysis
Entire study population	17523	−37.9 (−49.7 to −26.0)	<0.001	16273	−43.6 (−55.8 to −31.5)	<0.001	16273	−5.6 (−23.6 to 12.4)	0.54	15829	−7.4 (−19.6 to 4.8)	0.24
Women who did not change address	15191	−37.4 (−50.2 to −24.7)	<0.001	14196	−42.7 (−55.7 to −29.6)	<0.001	14196	−7.0 (−26.3 to 12.3)	0.48	13818	−4.7 (−17.8 to 8.4)	0.48
LMP-based GA only	16805	−35.4 (−47.5 to −23.2)	<0.001	15618	−40.8 (−53.3 to −28.4)	<0.001	15618	−3.2 (−21.6 to 15.1)	0.73	15195	−5.8 (−18.3 to 6.7)	0.36
Stratified analysis
Oslo	4669	7.5 (−27.7 to 42.7)	0.68	4380	−5.9 (−42.8 to 31.0)	0.75				4285	12.5 (−24.3 to 49.3)	0.51
Akershus	7547	10.5 (−22.8 to 43.9)	0.54	6982	8.9 (−25.4 to 43.1)	0.61				6759	29.2 (−4.8 to 63.1)	0.09
Bergen	3866	−15.6 (−43.7 to 12.4)	0.28	3577	−4.8 (−33.0 to 23.4)	0.74				3490	19.8 (−7.7 to 47.2)	0.16
Hordaland	1441	−37.6 (−104.6 to 29.4)	0.27	1334	−36.0 (−103.5 to 31.5)	0.30				1295	−26.7 (−92.7 to 39.2)	0.43
Not smoking	15440	−41.3 (−53.8 to −28.8)	<0.001	15229	−43.3 (−55.8 to −30.8)	<0.001	15229	−6.6 (−25.1 to 12.0)	0.49	14835	−5.6 (−18.2 to 6.9)	0.38
Smoking	1083	−28.3 (−80.0 to 23.3)	0.28	1044	−45.5 (−97.7 to 6.8)	0.09	1044	22.1 (−51.8 to 96.1)	0.56	994	−27.3 (−80.1 to 25.5)	0.31
Parity 0	8304	−16.8 (−33.3 to −0.4)	0.045	7803	−17.8 (−34.7 to −1.0)	0.04	7803	4.3 (−20.5 to 29.0)	0.74	7594	−8.3 (−25.2 to 8.5)	0.33
Parity 1	6326	−0.6 (−20.6 to 19.4)	0.95	5858	−6.9 (−27.4 to 13.5)	0.51	5858	21.8 (−8.2 to 51.8)	0.15	5695	2.0 (−18.3 to 22.4)	0.85
Parity ≥2	2893	−26.5 (−60.3 to 7.4)	0.13	2612	−31.0 (−66.4 to 4.4)	0.09	2612	17.8 (−31.7 to 67.4)	0.48	2540	−24.8 (−59.9 to 10.4)	0.17
Boys	8921	−30.7 (−47.5 to −13.8)	<0.001	8290	−39.6 (−57.0 to −22.2)	<0.001	8290	−7.5 (−33.0 to 18.1)	0.57	8040	−5.4 (−22.8 to 12.1)	0.55
Girls	8602	−45.5 (−62.0 to −29.1)	<0.001	7983	−47.8 (−64.8 to −30.8)	<0.001	7983	−3.6 (−28.9 to 21.8)	0.78	7789	−9.4 (−26.4 to 7.6)	0.28
Education less than high school	985	−35.4 (−95.3 to 24.5)	0.25	968	−24.5 (−83.4 to 34.5)	0.42	968	−18.4 (−96.7 to 60.0)	0.65	905	−27.8 (−87.2 to 31.5)	0.36
Education high school	4173	−31.9 (−58.5 to −5.3)	0.02	4098	−36.0 (−62.3 to −9.7)	0.007	4098	10.4 (−27.3 to 48.1)	0.59	3948	4.8 (−21.7 to 31.3)	0.72
Education up to 4 years of college	6474	−42.4 (−61.5 to −23.3)	<0.001	6403	−44.0 (−62.8 to −25.3)	<0.001	6403	−1.5 (−30.2 to 27.1)	0.92	6262	−4.9 (−23.7 to 13.9)	0.61
Education more than  4 years of college (master or professional degree)	4866	−48.2 (−69.6 to −26.9)	<0.001	4804	−50.2 (−71.4 to −29.0)	<0.001	4804	−17.8 (−49.4 to 13.8)	0.27	4714	−13.3 (−34.5 to 8.0)	0.22
Born in winter	4097	−20.2 (−46.6 to 6.2)	0.13	3797	−35.3 (−62.5 to −8.2)	0.01	3797	7.8 (−31.1 to 46.7)	0.69	3677	4.9 (−22.4 to 32.1)	0.73
Born in spring	4684	−60.6 (−82.2 to −39.0)	<0.001	4355	−60.2 (−82.2 to −38.3)	<0.001	4355	−46.7 (−79.5 to −13.8)	0.005	4226	−28.5 (−50.6 to −6.4)	0.01
Born in summer	4626	−35.1 (−57.4 to −12.8)	0.002	4272	−40.5 (−63.3 to −17.6)	0.001	4272	14.2 (−20.7 to 49.1)	0.43	4167	−2.7 (−25.7 to 20.3)	0.82
Born in autumn	4116	−28.8 (−54.9 to −2.7)	0.03	3849	−31.9 (−58.6 to −5.3)	0.02	3849	16.1 (−23.0 to 55.1)	0.42	3759	5.1 (−21.4 to 31.7)	0.70

Effect estimate in grams per 10 μg/m³ NO₂

GA gestational age, LMP last menstrual period

^aModel 1 adjusted for: maternal education, birth season, sex of child, maternal age, maternal marital status, maternal smoking during pregnancy, maternal height

^bModel 2 adjusted for: maternal education, birth season, sex of child, maternal age, maternal marital status, maternal smoking during pregnancy, maternal height, area

^cModel 3 adjusted for: maternal education, birth season, sex of child, maternal age, maternal marital status, maternal smoking during pregnancy, maternal height, parity, maternal weight; in stratified analysis the corresponding stratification variable is not included in the adjustment