Table 1 Characteristics of the overall study sample and by preterm birth status
|  | 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 (%) | Â | Â | Â | 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 (%) | Â | Â | Â | 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 (%) | Â | Â | Â | 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 (%) | Â | Â | Â | 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) |  |