Table 1 Baseline characteristics of the study participants

Right PI, median (IQR)

0.79 (0.66–0.96)

1.27 (0.84–1.63) ^a

0.83 (0.66–1.03) ^a

0.84 (0.67–1.08) ^a

Right RI, median (IQR)

0.51 (0.46–0.58)

0.67 (0.53–0.74) ^a

0.53 (0.46–0.61) ^a

0.53 (0.46–0.62) ^a

Left PI, median (IQR)

0.81 (0.67–0.98)

1..27 (0.87–1.73) ^a

0.86 (0.68–1.09) ^a

0.87 (0.69–1.12) ^a

Left RI, median (IQR)

0.52 (0.46–0.59)

0.64 (0.52–0.76) ^a

0.53 (0.46–0.62) ^a

0.54 (0.47–0.63) ^a

Gestational weeks, median (IQR)

39.0 (38.0–40.0)

33.0 (31.0–34.0) ^a

39.0 (38.0–39.0) ^a

38.0 (38.0–39.0) ^a

Local residence no.(%)

23,352 (79.1)

93 (78.8)

1375 (82.7) ^b

1468 (82.4) ^b

Maternal age (year) at delivery year no.(%)^a

 < 25

1336 (4.5)

2 (1.7)

59 (3.5)

61 (3.4)

 25–34

25,624 (86.8)

94 (79.7)

1374 (82.6)

1468 (82.4)

  ≥ 35

2549 (8.6)

22 (18.6)

230 (13.8)

252 (14.1)

 Nulliparous no. (%)

25,515 (86.5)

96 (81.4)

1504 (90.4) ^a

1600 (89.8) ^a

 Assisted conception no. (%)

442 (1.5)

6 (5.1) ^b

52 (3.1) ^a

58 (3.3) ^a

 Male fetus no. (%)

15,214 (51.6)

56 (47.5)

826 (49.7)

882 (49.5)