Table 2. Multiple linear regression model for birth weight in male and female infants.

Variables	Male (n = 4132)			Female (n = 4008)
	β	SE (β)	P-valuea	β	SE (β)	P-valuea
			(R2 = 0.42)			(R2 = 0.41)
Intercept	−2919.6	134.2	<0.001	−2932.0	133.9	<0.001
Smoking status, vs non-smokers			<0.001			<0.001
Ex-smokers who quit before pregnancy	−7.0	12.7		12.7	13.0
Ex-smokers who quit during early pregnancy	−27.8	16.3		−39.6	17.3
Current smokers	−136.4	24.3		−124.6	25.0
Primigravida	−107.0	11.1	<0.001	−117.5	11.5	<0.001
Partners’ smoking status			0.4			0.3
Ex-smokers who quit before pregnancy	−21.5	14.7		22.4	15.0
Ex-smokers who quit during early pregnancy	−39.3	33.2		3.5	34.2
Current smokers	−13.9	13.5		−3.8	13.8
Annual household income, %			0.6			0.4
<2 000 000 JPY	−3.5	33.9		−51.9	33.8
2 000 000–3 999 999 JPY	−14.0	27.5		−31.5	26.8
4 000 000–5 999 999 JPY	−30.8	27.2		−27.3	26.8
6 000 000–7 999 999 JPY	−8.3	28.8		−14.3	28.5
8 000 000–9 999 999 JPY	−16.8	33.1		5.3	32.8
Pregestational maternal weight status			<0.001			<0.001
Underweight (BMI <18.5)	−93.2	14.1		−85.5	14.7
Overweight (BMI ≥25.0)	157.3	17.7		172.8	17.8
Maternal weight gain during pregnancy, kg	21.4	1.4	<0.001	19.0	1.4	<0.001
Pregnancy-induced hypertension	−220.8	33.1	<0.001	−82.0	30.3	0.007
Diabetes mellitus or gestational diabetes	83.1	29.8	0.005	85.8	31.7	0.007
Maternal age group at delivery, %			0.2			0.3
<20 years	3.2	64.3		96.3	63.2
20–24 years	−0.1	20.0		−16.4	21.1
25–29 years	−20.5	13.2		8.9	13.5
35–39 years	−17.0	14.0		21.6	14.4
≥40 years	39.6	26.9		17.9	27.7
Gestational duration, days	21.4	0.5	<0.001	21.0	0.5	<0.001

BMI, body mass index; SE, standard error.

^aP-value was calculated using the t-test and the F-test.