The general view of the maternal
| Characteristics | Cohort population (n=24 833) | Validation population (n=5009) | P value |
| Age (years)，N (%) | 0.393 | ||
| <25 | 1,309 (5.27%) | 266 (5.31%) | |
| 25–29 | 11,736 (47.26%) | 2340 (46.71%) | |
| 30–34 | 9,445 (38.03%) | 1959 (39.11%) | |
| ≥35 | 2,343 (9.44%) | 444 (8.86%) | |
| Ethnicity，N (%) | 0.983 | ||
| Han | 22,222 (89.49%) | 4475 (89%) | |
| Manchu | 1,872 (7.54%) | 389 (7.8%) | |
| Other ethnic groups | 739 (2.98%) | 145 (2.9%) | |
| Educational attainment，N (%) | 0.115 | ||
| High school or below | 8,635 (34.77%) | 1742 (35%) | |
| Bachelor’s degree | 13,639 (54.92%) | 2703 (54%) | |
| Postgraduate or higher | 2,559 (10.30%) | 564 (11%) | |
| Occupation，N (%) | 0.777 | ||
| Unemployed | 11,373 (45.80%) | 2266 (45%) | |
| Light physical labour | 2,825 (11.38%) | 569 (11%) | |
| Moderate physical labour | 10,011 (40.31%) | 2038 (41%) | |
| Heavy physical labour | 624 (2.51%) | 136 (2.7%) | |
| Family per capita monthly income (10 000 yuan)，N (%) | 0.9862 | ||
| <0.5 | 10,325 (41.58%) | 2080 (42%) | |
| 0.5–2.0 | 9,534 (38.39%) | 1922 (38%) | |
| 2.0–5.0 | 3,584 (14.43%) | 720 (14%) | |
| >5.0 | 1,390 (5.60%) | 287 (5.7%) | |
| Prepregnancy BMI (Kg/m2)*，N (%) | <0.001 | ||
| <18.5 (underweight) | 7,294 (29.37%) | 1475 (29.45%) | |
| 18.5–23.9 (normal) | 15,005 (60.42%) | 2963 (59.16%) | |
| 24.0–27.9 (overweight) | 1,700 (6.85%) | 307 (6.13%) | |
| >28.0 (obesity) | 834 (3.36%) | 264 (5.29%) | |
| Pregnancy history，N (%) | 0.565 | ||
| 1 | 14,985 (60.34%) | 3005 (60%) | |
| 2 | 6,513 (26.23%) | 1303 (26%) | |
| ≥3 | 3,335 (13.43%) | 701 (14%) | |
| Parity (number of deliveries)，N (%) | 0.775 | ||
| 0 | 20,550 (82.75%) | 4127 (82%) | |
| 1 | 4,152 (16.72%) | 853 (17%) | |
| ≥2 | 131 (0.53%) | 29 (0.6%) | |
| Gestational age at delivery (weeks)，N (%) | 0.434 | ||
| <38 | 1,507 (6.07%) | 296 (5.91%) | |
| 38–40 | 13,023 (52.44%) | 2589 (51.59%) | |
| >40 | 10,303 (41.49%) | 2129 (42.50%) | |
| Blood loss (ml) | 393.54±92.53 | 413.48±124.65 | 0.081 |
| Postpartum haemorrhage，N (%) | 1,623 (6.54%) | 286 (5.71%) | 0.032 |
| Due to uterine atony，N (%) | 1,225 (4.93%) | 266 (5.31%) | 0.279 |
| Due to placental factors，N (%) | 242 (0.97%) | 43 (0.86%) | 0.489 |
| Due to cervical trauma，N (%) | 139 (0.56%) | 31 (0.62%) | 0.686 |
| Due to coagulation disorders，N (%) | 76 (0.31%) | 17 (0.34%) | 0.705 |
*p<0.05.