Table 1 Characteristics of the study population according to maternal rheumatoid arthritis before childbirth

Maternal characteristics

 Study country

  Denmark

4297

51.11

1,291,985

36.83

  Finland

215

2.56

912,370

26.00

  Sweden

3895

46.33

1,304,087

37.17

 Maternal age at delivery (years)

  < 25

735

8.74

539,085

15.37

  25–29

2271

27.02

1,116,203

31.81

  30–34

3163

37.62

1,188,931

33.89

   ≥ 35

2238

26.62

664,223

18.93

 Maternal country of origin ^a

  Denmark/Sweden

4262

52.03

1,494,124

57.55

  Other country

3929

47.96

1,099,794

42.37

  Unknown

 < 5

0.01

2154

0.08

 Maternal BMI in early pregnancy (kg/m²)^a

  < 18.5

204

2.49

57,841

2.23

  18.5–24.9

3728

45.51

1,084,402

41.76

  25.0–29.9

1654

20.19

475,472

18.32

  ≥ 30.0

876

10.69

235,133

9.06

  Unknown

1730

21.12

743,224

28.63

 Marital status at delivery

  Single, widowed or divorced

4710

56.03

1,761,697

50.21

  Married or in registered partnership

3690

43.89

1,712,614

48.82

  Unknown

7

0.08

34,131

0.97

 Maternal highest education at delivery

  Primary and lower secondary

1226

14.58

792,049

22.58

  Upper secondary

3449

41.03

1,235,290

35.21

  University

3668

43.63

1,334,893

38.05

  Unknown

64

0.76

146,210

4.16

 Maternal smoking in early pregnancy

  No

7122

84.72

2,954,987

84.23

  Yes

957

11.38

421,187

12.00

  Unknown

328

3.90

132,268

3.77

 Parity

  Primiparous

3533

42.02

1,441,227

41.08

  Multiparous

4874

57.98

2,066,8347 (58. 90)

58. 90

  Unknown

0

0.00

868

0.02

 Psychiatric disorders before childbirth

  No

7221

85.89

3,216,257

91.67

  Yes

1186

14.11

292,185

8.33

 Family history of psychiatric disorders^a

  No

6631

80.94

1,960,464

75.52

  Yes

1040

12.70

240,350

9.26

  Unknown ^b

521

6.36

395,258

15.24

 Diabetes

  No

7984

94.97

3,349,807

95.48

  Yes

423

5.03

158,735

4.52

 Pre-eclampsia

  No

7996

95.11

3,410,056

97.20

  Yes

411

4.89

98,386

2.80

 Placental abruption

  No

8363

99.48

3,496,287

99.65

  Yes

44

0.52

12,155

0.35

Characteristics of the birth

 Calendar year of birth

  1995–2000

759

9.03

566,190

16.14

  2001–2005

2077

24.70

1,048,250

29.88

  2006–2010

3017

35.89

1,119,437

31.90

  2011–2015

2554

30.38

774,565

22.08

 Small for gestational age

  No

7280

86.60

3,115,135

88.79

  Yes

1009

12.00

346,001

9.86

  Unknown

118

1.40

47,036

1.35

 Preterm birth (gestational < 37 weeks)

  No

7675

91.30

3,307,208

94.26

  Yes

667

7.93

170,470

4.86

  Unknown ^c

65

0.77

30,764

0.88

 Stillbirth

  No

8367

99.52

3,499,154

99.74

  Yes

40

0.48

9288

0.26