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Table 1 Characteristics of confounding factors for preterm birth (PTB) cases

From: Fermented foods and preterm birth risk from a prospective large cohort study: the Japan Environment and Children’s study

  Preterm delivery Non preterm delivery p value
(n = 2343) (n = 75,324)
Age, n (%) < 0.001
 < 30 years old 867 (37.0) 29,388 (39.0)
 30–34 years old 796 (34.0) 26,841 (35.6)
 ≥ 35 years old 680 (29.0) 19,092 (25.4)
BMI(kg/m2) group, n (%) < 0.001
 Leptosome (< 18.5) 488 (20.8) 12,272 (16.3)
 Healthy weight(18.5 to < 25) 1614 (68.9) 56,093 (74.5)
 Obese (≥ 25) 240 (10.3) 6924 (9.2)
Smoking history, n (%) 0.083
 Yes 999 (43.1) 30,862 (41.3)
 No 1321 (56.9) 43,931 (58.7)
Nulliparity, n (%) 0.022
 Yes 1015 (43.6) 30,927 (41.2)
 No 1314 (56.4) 44,135 (58.8)
Educational background (years), n (%) 0.028
 < 10 132 (5.9) 3410 (4.6)
 10–12 748 (33.4) 24,652 (33.2)
 13–15 883 (39.4) 29,886 (40.2)
 ≥ 16 476 (21.3) 16,417 (22.1)
Household income (million Japanese Yen/year), n (%) 0.032
 < 2 139 (6.6) 3769 (5.4)
 2 to < 4 706 (33.7) 24,147 (34.8)
 4 to < 6 728 (34.7) 22,976 (33.1)
 6 to < 8 303 (14.5) 11,068 (15.9)
 8 to < 10 125 (6.0) 4577 (6.6)
 ≥ 10 95 (4.4) 2947 (4.2)
Working > 42 h/week, n (%) 0.691
 Yes 481 (20.5) 15,211 (20.2)
 No 1862 (79.5) 60,113 (79.8)
Part-timer, n (%) 0.863
 Yes 371 (15.8) 12,027 (16.0)
 No 1972 (84.2) 63,297 (84.0)
  1. Note: BMI; body mass index before pregnancy