Table 1.
Basic characteristics of study populations. Values are numbers (percentages) unless stated otherwise
| Variable | Derivation cohort (England) (n=433 353) | Validation cohort (Sweden) (n=662 387) |
|---|---|---|
| Venous thromboembolism events | 315 (0.07) | 521 (0.08) |
| Social and demographic factors: | ||
| Mean (SD) age at delivery, years | 29.38 (5.90) | 30.32 (5.23) |
| Mean (SD) body mass index* | 24.05 (4.90) | 24.62 (4.57) |
| Smoker† (latest record before delivery) | 93 264 (21.52) | 32 502 (4.91) |
| Deliveries in 2004 or thereafter | 280 498 (64.73) | 662 387 (100) |
| Comorbidities‡: | ||
| Varicose veins | 10 935 (2.52) | 5156 (0.78) |
| Heart disease | 4431 (1.02) | 5072 (0.77) |
| Kidney disease | 4168 (0.96) | 6666 (1.01) |
| Inflammatory bowel disease | 2126 (0.49) | 5285 (0.80) |
| Pregnancy complications: | ||
| Pre-eclampsia/eclampsia | 9966 (2.30) | 24 013 (3.63) |
| Diabetes§ | 14 604 (3.37) | 14 948 (2.26) |
| Hypertension§ | 41 300 (9.53) | 7980 (1.20) |
| Parity: | ||
| Nulliparous¶ | 244 233 (56.36) | 293 176 (44.26) |
| 1 | 130 121 (30.03) | 242 341 (36.59) |
| 2 | 38 599 (8.91) | 88 803 (13.41) |
| ≥3 | 20 400 (4.71) | 38 067 (5.75) |
| Delivery characteristics/complications: | ||
| Preterm birth (<37 weeks) | 31 526 (7.27) | 31 728 (4.79) |
| Postpartum haemorrhage | 42 978 (9.92) | 48 383 (7.30) |
| Spontaneous/assisted vaginal delivery | 328 416 (75.78) | 547 654 (82.68) |
| Elective caesarean section | 44 143 (10.19) | 58 012 (8.76) |
| Emergency caesarean section | 60 794 (14.03) | 56 721 (8.56) |
| Multiple delivery (twins or more) | 6550 (1.51) | 9308 (1.41) |
| Stillbirth | 1972 (0.46) | 2286 (0.35) |
| Puerperal acute infection | 13 681 (3.16) | 48 383 (7.30) |
| Infant’s mean (SD) birth weight**, g | 3368.35 (596.80) | 3519.80 (581.9) |
| Missing information: | ||
| Infant birth weight | 87 305 (20.14[20.15?]) | 930 (0.14) |
| Body mass index | 98 868 (22.81) | 57 173 (8.63) |
*Pregnancies with missing pre-pregnancy body mass index: 23% (England), 8.6% (Sweden).
†Latest smoking record before delivery.
‡Comorbidities recorded ever before delivery; for specific disease group for comorbidities, see supplementary table A.
§Including gestational and pre-existing.
¶Antenatal parity.
**Pregnancies with missing infant birthweight information: 20% (England), 0.1% (Sweden).