Table 1 Background characteristics of study participants (N=796)

Children’s characteristics

 Birth order, n (%)

  First

401

50.4

  Second or higher

395

49.6

 Gender, n (%)

  Girls

392

49.2

  Boys

404

50.8

 Gestational age at birth in week, mean (SD)

38.9

1.52

 Preterm birth, n (%)

  Yes

51

6.4

  No

745

93.6

 Birth weight (g), mean (SD)

2949.3

440.1

  <2000 g

19

2.4

  2000–2499 g

82

10.3

  ≥2500 g

695

87.3

 ADHD-PRS, mean (SD)

0.0022

1.01

 ADHD-RS total score, median (IQR)

5^a

1–10^b

  Inattention, median (IQR)

3^a

1–7^b

  Hyperactivity, median (IQR)

1^a

0-3^b

Parent’s characteristics

 Mother’s age at delivery, n (%)

  <35 years

567

71.2

  ≥35 years

229

28.8

 Maternal educational attainment, n (%)

  ≤12 years

246

30.9

  >12 years

550

69.1

 Maternal pre-pregnancy BMI, n (%)

  Underweight (<18.5)

170

21.3

  Normal weight (18.5-24.9)

541

68.0

  Overweight (≥25.0)

85

10.7

 Pre-pregnancy or during pregnancy smoking history, n (%)

  Yes

155

19.5

  No

641

80.5

 Alcohol consumption during pregnancy, n (%)

  Yes

110

13.8

  No

686

86.2

 Father’s age at birth

  <25 years

45

5.6

  25–34 years

456

57.3

  ≥35 years

295

37.1

 Father’s education at birth

  ≤12 years

59

7.4

  >12 years

737

92.6

 Household annual income at birth in JPY

  ≤3 million

36

4.5

  3–8 million

604

75.9

  ≥8 million

156

19.6