Table 1 Characteristics of the participants.

Age at early pregnancy, year

 < 0.0001

 < 25

202

(7.0)

5546

(8.6)

25 to < 30

716

(24.8)

18,681

(29.0)

30 to < 35

1019

(35.3)

23,518

(36.5)

 ≥ 35

948

(32.9)

16,652

(25.9)

Pre-pregnancy body mass index, kg/m²

< 0.0001

 < 18.5

535

(18.5)

10,160

(15.8)

18.5 to < 25

1966

(68.2)

47,946

(74.5)

 ≥ 25

384

(13.3)

6291

(9.8)

Parity

0.2226

Primipara

1223

(42.4)

28,040

(43.5)

Multipara

1662

(57.6)

36,357

(56.5)

History of maternal allergy

0.0105

No

1496

(51.9)

31,825

(49.4)

Yes

1389

(48.2)

32,572

(50.6)

Physical activity corresponding to over 10-minute walk/day

 < 0.0001

No

865

(30.0)

15,161

(23.5)

Yes

2020

(70.0)

49,236

(76.5)

Quartile of energy intake during pregnancy

0.1215

Q1 (≤ 1300 kcal/day)

717

(24.9)

15,362

(23.9)

Q2 (1300–1614 kcal/day)

686

(23.8)

16,576

(25.7)

Q3 (1615–2009 kcal/day)

747

(25.9)

16,380

(25.4)

Q4 (≥ 2010 kcal/day)

735

(25.5)

16,079

(25.0)

Alcohol intake

0.0007

Never

1057

(36.6)

21,525

(33.4)

Former

1765

(61.2)

41,117

(63.9)

Current

63

(2.2)

1755

(2.7)

Active smoking history

0.0046

Never

1708

(59.2)

38,872

(60.4)

Former

1046

(36.2)

23,327

(36.2)

Current

131

(4.5)

2198

(3.4)

Passive smoking history

0.0010

Never

1857

(64.4)

41,592

(64.6)

1–6 days /week

654

(22.7)

15,753

(24.5)

Everyday

374

(13.0)

7052

(11.0)

Annual household income, million JPY

0.2177

 < 4

1149

(39.8)

24,718

(38.4)

4 to < 6

965

(33.5)

21,671

(33.7)

 ≥ 6

771

(26.7)

18,008

(28.0)

Highest education level, year

0.4720

 < 12

972

(33.7)

21,071

(32.7)

12 to < 16

1246

(43.2)

27,944

(43.4)

 ≥ 16

667

(23.1)

15,382

(23.9)

Employed during early pregnancy

0.1652

No

1343

(46.5)

29,131

(45.2)

Yes

1542

(53.5)

35,266

(54.8)

Marital status

0.0001

Married

2773

(96.1)

62,167

(96.5)

Single

75

(2.6)

1823

(2.8)

Divorced or widowed

37

(1.3)

407

(0.6)

Caesarean section

 < 0.0001

No

1658

(57.5)

53,163

(82.6)

Yes

1227

(42.5)

11,234

(17.4)

Child sex

 < 0.0001

Boy

1699

(58.9)

32,766

(50.9)

Girl

1186

(41.1)

31,631

(49.1)

Major anomaly

 < 0.0001

No

2718

(94.2)

63,086

(98.0)

Yes

167

(5.8)

1311

(2.0)

Birth season

0.1057

Spring (March–May)

684

(23.7)

15,054

(23.4)

Summer (June–August)

804

(27.9)

17,109

(26.6)

Autumn (September–November)

736

(25.5)

17,723

(27.5)

Winter (December–February)

661

(22.9)

14,511

(22.5)

Feeding methods during the first month after birth

 < 0.0001

Breastfeeding only

891

(30.9)

28,038

(43.5)

Mixed feeding

1927

(66.8)

35,621

(55.3)

Infant formula only

67

(2.3)

738

(1.2)

Attending a childcare facility at 6 months of age

0.1511

No

2714

(94.1)

60,144

(93.4)

Yes

171

(5.9)

4253

(6.6)

Palivizumab administration

< 0.0001

Yes

1169

(41.7)

504

(0.8)

No or unsure

1635

(58.3)

61,775

(99.2)