Table 1 Baseline characteristics

Age (years)^a

31.0 (29.0–33.0)

30.0 (27.0–33.0)

31.0 (29.0–33.0)

 Women under 20 years old

5 (0.5)

4 (2.6)

1 (0.1)

 Women between 20–29 years old

362 (33.8)

58 (37.4)

304 (33.2)

 Women between 30–39 years old

686 (64.0)

93 (60.0)

593 (64.7)

 Women above 40 years old

19 (1.7)

0 (0.0)

19 (2.1)

 Missing data

5

1

4

Ethnic background

 Women with a Dutch ethical background

1,013 (94.7)

141 (91.6)

872 (95.2)

 Women with a non-Dutch ethical background

57 (5.3)

13 (8.4)

44 (4.8)

 Missing data

7

2

5

Pre-pregnancy BMI^a

23.2 (21.3–26.4)

23.5 (21.0–27.3)

23.2 (21.4–26.1)

 Women who are underweight (< 18.5 kg/m²)

20 (1.9)

5 (3.2)

15 (1.6)

 Women with a healthy weight range (18.5—24.9 kg/m²)

699 (65.5)

88 (57.1)

611 (66.9)

 Women who are overweight (25.0—29.9 kg/m²)

218 (20.4)

41 (26.6)

177 (19.4)

 Women with obesity (≥ 30.0 kg/m²)

130 (12.2)

20 (13.0)

110 (12.0)

 Missing

10

2

8

Level of education

 Women with a low or moderate level of education

307 (28.7)

64 (41.6)

243 (26.5)

 Women with a high level of education

763 (71.3)

90 (58.4)

673 (73.5)

 Missing

7

2

5

Parity

 Women who are nullipara

508 (47.4)

62 (40.0)

446 (48.6)

 Women who are multipara

564 (52.6)

93 (60.0)

471 (51.4)

 Missing

5

1

4

Time to conception

3.0 (1.0 – 6.0)

1.0 (1.0–3.5)

3.0 (1.0–7.0)

 < 6 months

722 (70.3)

91 (80.5)

631 (96.0)

 6—11 months

153 (14.9)

12 (10.6)

141 (15.5)

 ≥ 12 months

152 (14.8)

10 (8.8)

142 (15.5)

 Missing

50

43

7