Table 1 Characteristics of the study population

Maternal age at pregnancy, years, Mean ± SD

29.05 ± 5.60

29.11 ± 5.97

29.09 ± 5.56

28.07 ± 5.47

F = 2267.709

< 0.001

Maternal race, n (%)

χ² = 11,577.64

< 0.001

 White

2,300,082 (74.19)

199,429 (68.10)

1,993,838 (74.78)

106,815 (75.84)

 Black

471,532 (15.21)

63,396 (21.65)

387,968 (14.55)

20,168 (14.32)

 Asian

207,156 (6.68)

17,141 (5.85)

182,473 (6.84)

7542 (5.35)

 Others

121,352 (3.91)

12,891 (4.40)

102,135 (3.83)

6326 (4.49)

Maternal education, n (%)

χ² = 16,491.96

< 0.001

  < 12 grade

347,915 (11.22)

44,010 (15.03)

285,328 (10.70)

18,577 (13.19)

 High school or GED

819,024 (26.42)

89,561 (30.58)

687,805 (25.80)

41,658 (29.58)

 Some college

884,826 (28.54)

84,866 (28.98)

758,856 (28.46)

41,104 (29.18)

 Bachelor or above

1,048,357 (33.82)

74,420 (25.41)

934,425 (35.04)

39,512 (28.05)

Pre-pregnancy BMI, n (%)

χ² = 2585.980

< 0.001

 Underweight

95,608 (3.08)

10,951 (3.74)

80,502 (3.02)

4155 (2.95)

 Normal

1,297,882 (41.87)

113,921 (38.90)

1,127,107 (42.27)

56,854 (40.36)

 Overweight

844,091 (27.23)

77,880 (26.59)

728,318 (27.31)

37,893 (26.90)

 Obesity

862,541 (27.82)

90,105 (30.77)

730,487 (27.40)

41,949 (29.78)

GWG, kg, M (Q₁, Q₃)

13.17 (9.08, 17.25)

11.35 (7.26, 15.89)

13.17 (9.08, 17.25)

13.62 (9.08, 17.71)

χ² = 15,994.35#

< 0.001

GWG z-score, M (Q₁, Q₃)

0.44 (0.28, 0.57)

0.52 (0.35, 0.69)

0.44 (0.28, 0.57)

0.40 (0.23, 0.52)

χ² = 43,068.24#

< 0.001

Smoking before pregnancy, n (%)

χ² = 3930.086

< 0.001

 No

2,862,009 (92.32)

262,842 (89.75)

2,471,714 (92.70)

127,453 (90.49)

 Yes

238,113 (7.68)

30,015 (10.25)

194,700 (7.30)

13,398 (9.51)

Smoking during pregnancy, n (%)

χ² = 4932.560

 < 0.001

 No

2,941,838 (94.89)

270,492 (92.36)

2,539,377 (95.24)

131,969 (93.69)

 Yes

158,284 (5.11)

22,365 (7.64)

127,037 (4.76)

8882 (6.31)

Number of prenatal visits, M (Q₁, Q₃)

12.00 (9.00, 13.00)

10.00 (7.00, 12.00)

12.00 (10.00, 13.00)

12.00 (10.00, 14.00)

χ² = 68,358.24#

< 0.001

Prior other terminations, n (%)

χ² = 1352.827

< 0.001

 No

2,258,968 (72.87)

205,929 (70.32)

1,947,264 (73.03)

105,775 (75.10)

 Yes

841,154 (27.13)

86,928 (29.68)

719,150 (26.97)

35,076 (24.90)

History of preterm birth, n (%)

χ² = 31,201.89

< 0.001

 No

2,995,626 (96.63)

266,590 (91.03)

2,591,520 (97.19)

137,516 (97.63)

 Yes

104,496 (3.37)

26,267 (8.97)

74,894 (2.81)

3335 (2.37)

WIC food during pregnancy, n (%)

χ² = 5200.329

< 0.001

 No

2,064,986 (66.61)

179,731 (61.37)

1,796,666 (67.38)

88,589 (62.90)

 Yes

1,035,136 (33.39)

113,126 (38.63)

869,748 (32.62)

52,262 (37.10)

Gestational age, Mean ± SD

38.73 ± 2.01

34.38 ± 2.18

39.00 ± 1.11

42.58 ± 0.74

F = 2,563,966

< 0.001

Gender of newborn, n (%)

χ² = 801.768

< 0.001

 Female

1,513,848 (48.83)

136,304 (46.54)

1,306,395 (48.99)

71,149 (50.51)

 Male

1,586,274 (51.17)

156,553 (53.46)

1,360,019 (51.01)

69,702 (49.49)

Multipara, n (%)

χ² = 1769.685

< 0.001

 No

1,178,778 (38.02)

104,300 (35.61)

1,015,028 (38.07)

59,450 (42.21)

 Yes

1,921,344 (61.98)

188,557 (64.39)

1,651,386 (61.93)

81,401 (57.79)

Eclampsia, n (%)

χ² = 4606.815

< 0.001

 No

3,092,573 (99.76)

290,422 (99.17)

2,661,519 (99.82)

140,632 (99.84)

 Yes

7549 (0.24)

2435 (0.83)

4895 (0.18)

219 (0.16)

Hypertension during pregnancy, n (%)

χ² = 30,026.00

< 0.001

 No

2,865,842 (92.44)

247,322 (84.45)

2,485,062 (93.20)

133,458 (94.75)

 Yes

234,280 (7.56)

45,535 (15.55)

181,352 (6.80)

7393 (5.25)