Table 1 Background demographics of the first-time mothers in the IA and non-IA groups at the beginning of the on-going pregnancy

Age

<20 y

466

9.02

4,578

8.76

<0.0001

20–24 y

1,555

30.09

12,416

23.77

25–29 y

1,683

32.57

19,477

37.28

30–34 y

1,008

19.51

11,850

22.68

35–39 y

375

7.26

3,368

6.45

≥40 y

80

1.55

550

1.05

Living area

Urban

3,910

77.47

39,376

77.01

0.2188

Rural

484

9.59

5,292

10.35

Densely populated community

653

12.94

6,461

12.64

Cohabiting

Yes

4,039

78.40

45,312

86.89

<0.0001

No

661

12.83

3,588

6.88

Smoking

Non-smoking

3,168

62.33

42,794

83.56

<0.0001

Stopped in first trim

652

12.83

3,562

6.96

Smokes after first trim

1,263

24.85

4,856

9.48

BMI (kg/m²)

Underweight (<20)

293

5.67

3,548

6,79

<0.0001

Normalweight (20–25)

3,233

62.57

33,949

64.99

Overweight (>25)

1,641

31.76

14,742

28.22