Table 1 Study participants characteristics with and without oral contraceptive use in the 12 months before conception

N

%

N

%

Age at childbirth

   < 20

10

2.9

5

2.2

   20–24

67

19.7

82

35.0

   25–29

146

42.8

104

44.4

   ≥ 30

118

34.6

43

18.4

Education

   ≤ High school

113

33.6

86

37.1

   Some college

134

39.9

114

49.1

   ≥ College graduate

89

26.5

32

13.8

   Missing

5

2

Birth order

   1

166

48.7

147

62.8

   2

126

36.9

76

32.5

   ≥ 3

49

14.4

11

4.7

Menarche

   ≤ 11 years old

38

11.3

39

16.7

   12–15 years old

280

83.3

183

78.2

   ≥ 16 years old

18

5.4

12

5.1

   Missing

5

0

Smoking during pregnancy

   Yes

85

24.9

88

37.6

   No

256

75.1

146

62.4

Planned pregnancy

   Yes

234

68.6

187

79.9

   Undecided

35

10.3

16

6.8

   No

72

21.1

31

13.3