Table 1. Basic information according to maternal age.

	Total (n = 38,412)	20–24 years (n = 5,524)	25–29 years (n = 13,065)	30–34 years (n = 11,992)	35–39 years (n = 6,381)	$⩾$40 years (n = 1,450)	P
Maternal medical background
K6 score at the first trimester, mean (SD)	3.9 (4.0)	4.8 (4.5)	4.1 (4.0)	3.7 (3.8)	3.4 (3.6)	3.1 (3.3)	<0.001a
K6 score ≥13 at the first trimester, %	3.9	6.9	4.3	3.1	2.7	1.3	<0.001b
Smoked during pregnancy, %	3.6	7.2	3.5	2.8	2.9	2.4	<0.001b
Socioeconomic background
Annual household income, %
<2,000,000 JPY	5.2	16.2	4.7	3.0	2.9	2.9	<0.001b
2,000,000–5,999,999 JPY	66.0	73.2	71.0	63.1	58.6	54.3
6,000,000–9,999,999 JPY	24.2	8.4	21.5	28.9	31.1	32.5
≥10,000,000 JPY	4.5	2.2	2.8	5.1	7.4	10.3
Maternal education, %
<10 years	3.1	9.7	3.1	1.4	1.3	1.4	<0.001b
10–12 years	29.1	53.9	27.5	22.2	24.3	28.7
13–16 years	42.9	30.9	44.2	44.2	46.8	47.5
≥17 years	24.9	5.5	25.2	32.2	27.7	22.4
Obstetric outcome
Gestational age, weeks, mean (SD)	39.0 (1.8)	39.1 (1.6)	39.1 (1.6)	39.0 (1.8)	38.8 (2.0)	38.7 (2.0)	<0.001a
PTB before 37 weeks, %	5.3	4.3	4.5	5.6	6.7	8.3	<0.001b
PTB before 34 weeks, %	1.4	0.9	1.0	1.5	1.9	2.6	<0.001b
Birth weight, g, mean (SD)	2,980 (439)	3,001 (412)	2,991 (415)	2,979 (447)	2,952 (476)	2,923 (504)	<0.001a
LBW <2,500 g, %	10.2	8.4	9.1	10.4	12.8	14.7	<0.001b
LBW <1,500 g, %	0.9	0.6	0.7	1.0	1.4	1.8	<0.001b
SGA, %	5.6	4.9	5.1	5.7	6.7	7.6	<0.001b
Weight of placenta, g, mean (SD)	556 (121)	561 (138)	557 (111)	554 (122)	554 (122)	547 (124)	<0.001a

JPY, Japanese yen; LBW, low birth weight; PTB, preterm birth; SD, standard deviation.

^aP-value, one-way analysis of variance.

^bP-value, chi-square test.