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Table 1 Characteristics of the overall study sample and by preterm birth status

From: Association between maternal outdoor physical exercise and the risk of preterm birth: a case-control study in Wuhan, China

  Overall (n = 6656) Full-term (n = 4263) Preterm (n = 2393) P-values
Exercise per day, in minutes, mean ± SD 43.4 ± 24.3 44.0 ± 24.6 42.1 ± 23.6 0.012
Exercise frequency per week, no (%)     < 0.001
 Zero 1881 (28.3) 1107 (26.0) 774 (32.3)  
 1–2 1285 (19.3) 834 (19.6) 451 (18.8)  
 3–4 1179 (17.7) 802 (18.8) 377 (15.8)  
  > 5 2170 (32.6) 1476 (34.6) 694 (29.0)  
 Doctors suggested not to 141 (2.1) 44 (1.0) 97 (4.1)  
Maternal age, no (%)     < 0.001
  < 20 years 1148 (17.2) 618 (14.5) 530 (22.1)  
 20 to < 25 years 2966 (44.6) 1902 (44.6) 1064 (44.5)  
 25 to < 30 years 2213 (33.2) 1529 (35.9) 684 (28.6)  
  ≥ 30 years 329 (4.9) 214 (5.0) 115 (4.8)  
Education, no (%)     0.112
 Less than high school 1279 (19.2) 825 (19.4) 454 (19.0)  
 High school or occupational school 1917 (28.8) 1262 (29.6) 655 (27.4)  
 College 3131 (47.0) 1960 (46.0) 1171 (48.9)  
 Graduate school 329 (4.9) 216 (5.1) 113 (4.7)  
Income, no (%)     0.058
 Middle income (3000–6999 RMB) 4482 (67.3) 2910 (68.3) 1572 (65.7)  
 Low income (< 3000 RMB) 626 (9.4) 379 (8.9) 247 (10.3)  
 High income (> = 7000 RMB) 1548 (23.3) 974 (22.8) 574 (24.0)  
Family history, no (%) 297 (4.5) 129 (3.0) 168 (7.0) < 0.001
Neonatal gender: male, no (%) 3699 (55.6) 2273 (53.3) 1426 (59.6) < 0.001
Parity: more than one, no (%) 1119 (16.8) 682 (16.0) 437 (18.3) 0.02
BMI prior to pregnancy, no (%)     0.149
 Underweight 1325 (19.9) 869 (20.4) 456 (19.1)  
 Normal 4933 (74.1) 3154 (74.0) 1779 (74.3)  
 Overweight/Obesity 398 (6.0) 240 (5.6) 158 (6.6)  
Weight gain in kilograms, mean ± SD 14.3 ± 4.0 14.5 ± 4.0 13.8 ± 3.8 < 0.001
Maternal comorbidity, no (%) 499 (7.5) 248 (5.8) 251 (10.5) < 0.001
Anemia, no (%) 684 (10.3) 439 (10.3) 245 (10.2) 0.972
Colporrhagia, no (%) 1668 (25.1) 865 (20.3) 803 (33.6) < 0.001
Indoor air pollution, no (%) 1060 (15.9) 671 (15.7) 389 (16.3) 0.605
District, no (%)     0.047
 Hanyang 997 (15.0) 634 (14.9) 363 (15.2)  
 Hongshan 967 (14.5) 640 (15.0) 327 (13.7)  
 Jiangan 1256 (18.9) 760 (17.8) 496 (20.7)  
 Jianghan 821 (12.3) 527 (12.4) 294 (12.3)  
 Qiaokou 933 (14.0) 588 (13.8) 345 (14.4)  
 Qingshan 510 (7.7) 337 (7.9) 173 (7.2)  
 Wuchang 1172 (17.6) 777 (18.2) 395 (16.5)  
  1. Maternal comorbidity includes any of the following comorbidities: diabetes, thyroid disease, hepatitis, hypertension, heart disease, renal disease, epilepsy, asthma, depression, anxiety, or others
  2. SD standard deviation