Table 1 Characteristics of participants

 Vaginal

3267 (74.9%)

396 (77.5%)

88 (86.3%)

 Cesarean

1097 (25.1%)

115 (22.5%)

14 (13.7%)

 2015

1674 (38.3%)

210 (41.1%)

49 (48.0%)

 2016

1540 (35.3%)

185 (36.2%)

39 (38.2%)

 2017

1150 (26.4%)

116 (22.7%)

14 (13.7%)

 Spring

976 (22.4%)

115 (22.5%)

22 (21.6%)

 Summer

1001 (22.9%)

115 (22.5%)

22 (21.6%)

 Autumn

1158 (26.5%)

122 (23.9%)

26 (25.5%)

 Winter

1229 (28.2%)

159 (31.1%)

32 (31.4%)

Maternal age

26.7 ± 3.6

26.6 ± 3.5

26.3 ± 3.6

 18–19

66 (1.5%)

7 (1.4%)

4 (3.9%)

 20–24

1212 (27.8%)

135 (26.4%)

25 (24.5%)

 25–29

2129 (48.8%)

272 (53.2%)

56 (54.9%)

 30–35

957 (21.9%)

97 (19.0%)

17 (16.7%)

 0

3751 (85.9%)

467 (91.4%)

96 (94.1%)

  ≥ 1

613 (14.1%)

44 (8.6%)

6 (5.9%)

 No

4045 (92.7%)

474 (92.8%)

95 (93.1%)

 Yes

319 (7.31%)

37 (7.2%)

7 (6.9%)

 No

4308 (98.7%)

490 (95.9%)

90 (88.2%)

 Yes

56 (1.3%)

21 (4.1%)

12 (11.8%)

 No

4175 (95.7%)

502 (98.2%)

101 (99.0%)

 Yes

189 (4.3%)

9 (1.8%)

1 (1.0%)

 No

4173 (95.6%)

493 (96.5%)

100 (98.0%)

 Yes

191 (4.4%)

18 (3.5%)

2 (2.0%)

Mean temperature across the whole pregnancy (°C)

18.0 ± 2.3

18.0 ± 2.3

18.0 ± 2.4

Mean concentration of PM2.5 in the 1st trimester (μg/m³)

63.7 (47.6–85.6)

64.4 (50.9–84.5)

65.3 (53.2–86.9)

Mean concentration of PM2.5 in the 2nd trimester (μg/m³)

59.3 (41.7–81.4)

59.2 (41.8–81.0)

59.5 (42.0–82.3)

Mean concentration of PM2.5 in the 3rd trimester (μg/m³)

55.8 (39. 9–73.8)

55.0 (40.1–79.0)

59.2 (43.7–79.0)

Mean concentration of PM2.5 across the whole pregnancy (μg/m³)

61.8 (55.4–67.8)

62.7 (55.8–68.0)

65.9 (56.2–70.5)