Sociodemographic, pre-existing and pregnancy-related clinical characteristics of women
| Cases n=1041 | Controls n=1984 | Unadjusted analysis | |||||
| n | % | n | % | OR | (95% CI) | P value | |
| Maternal age (years) | 0.30 | ||||||
| Under 20 | 43 | 4.1 | 55 | 2.8 | 1.55 | (0.98 to 2.46) | |
| 20–24 | 177 | 17.0 | 325 | 16.4 | 1.08 | (0.85 to 1.37) | |
| 25–29 | 312 | 30.0 | 619 | 31.2 | 1 | ||
| 30–34 | 342 | 32.9 | 628 | 31.7 | 1.08 | (0.89 to 1.32) | |
| 35–39 | 155 | 14.9 | 324 | 16.3 | 0.95 | (0.74 to 1.22) | |
| ≥40 | 12 | 1.2 | 33 | 1.7 | 0.72 | (0.40 to 1.30) | |
| Missing | 0 | 0 | |||||
| Ethnic group | 0.004 | ||||||
| White (UK and Ireland) | 641 | 61.6 | 1210 | 61.0 | 1 | ||
| White (other) | 170 | 16.3 | 246 | 12.4 | 1.30 | (1.05 to 1.62) | |
| Asian | 140 | 13.5 | 289 | 14.6 | 0.91 | (0.76 to 1.10) | |
| Black | 51 | 4.9 | 134 | 6.8 | 0.72 | (0.50 to 1.03) | |
| Other | 39 | 3.8 | 105 | 5.3 | 0.70 | (0.46 to 1.06) | |
| Missing | 0 | 0 | |||||
| Socioeconomic status | 0.06 | ||||||
| Higher managerial, admin, prof | 301 | 28.9 | 540 | 27.2 | 1 | ||
| Intermediate | 202 | 19.4 | 362 | 18.3 | 1.00 | (0.81 to 1.24) | |
| Routine and manual | 237 | 22.8 | 499 | 25.2 | 0.85 | (0.69 to 1.06) | |
| Unemployed/Student | 100 | 9.6 | 160 | 8.1 | 1.12 | (0.88 to 1.43) | |
| Employed, job unrecorded or uncodable | 66 | 6.3 | 123 | 6.2 | 0.96 | (0.70 to 1.33) | |
| Employment status not recorded | 135 | 13.0 | 300 | 15.1 | 0.81 | (0.65 to 1.00) | |
| Area deprivation quintile* | 0.29 | ||||||
| First (least deprived) | 175 | 17.0 | 374 | 19.0 | 1 | ||
| Second | 192 | 18.6 | 390 | 19.9 | 1.05 | (0.82 to 1.35) | |
| Third | 211 | 20.5 | 409 | 20.8 | 1.10 | (0.87 to 1.40) | |
| Fourth | 242 | 23.5 | 408 | 20.8 | 1.27 | (0.98 to 1.64) | |
| Fifth (most deprived) | 210 | 20.4 | 383 | 19.5 | 1.17 | (0.92 to 1.49) | |
| Missing | 11 | 20 | |||||
| Smoking status | 0.71 | ||||||
| Non-smoker during pregnancy | 903 | 86.9 | 1720 | 86.7 | 1 | ||
| Smoker during pregnancy | 125 | 12.0 | 235 | 11.8 | 1.01 | (0.80 to 1.28) | |
| Not recorded | 11 | 1.1 | 29 | 1.5 | 0.72 | (0.32 to 1.61) | |
| Missing | 2 | 0 | |||||
| Previous pregnancies ≥24 weeks | <0.0001 | ||||||
| 0 | 523 | 50.2 | 704 | 35.5 | 1.98 | (1.70 to 2.31) | |
| 1 | 328 | 31.5 | 875 | 44.1 | 1 | ||
| 2 | 137 | 13.2 | 280 | 14.1 | 1.31 | (1.03 to 1.65) | |
| 3 or more | 53 | 5.1 | 125 | 6.3 | 1.13 | (0.77 to 1.67) | |
| Missing | 0 | 0 | |||||
| BMI at booking (kg/m2) | 0.44 | ||||||
| <18.5 | 23 | 2.2 | 56 | 2.8 | 0.73 | (0.42 to 1.26) | |
| 18.5–24.9 | 529 | 50.8 | 1063 | 53.6 | 0.88 | (0.75 to 1.05) | |
| 25–29.9 | 296 | 28.4 | 526 | 26.5 | 1 | ||
| 30–35.0 | 89 | 8.6 | 154 | 7.8 | 1.03 | (0.76 to 1.38) | |
| >35.0 | 26 | 2.5 | 34 | 1.7 | 1.36 | (0.78 to 2.37) | |
| Not recorded | 78 | 7.5 | 151 | 7.6 | 0.92 | (0.73 to 1.16) | |
| Missing | 0 | 0 | |||||
| Pre-existing medical risk factors† | 0.69 | ||||||
| None | 1026 | 98.6 | 1956 | 98.7 | 1 | ||
| One or more | 15 | 1.4 | 25 | 1.3 | 1.14 | (0.59 to 2.21) | |
| Missing | 0 | 3 | |||||
| Problems in a previous pregnancy‡ (multiparous women only) | 0.03 | ||||||
| None | 494 | 95.4 | 1247 | 97.4 | 1 | ||
| One or more | 24 | 4.6 | 33 | 2.6 | 1.83 | (1.05 to 3.20) | |
| Missing | 0 | 0 | |||||
| Current pregnancy maternal problem§ | 0.07 | ||||||
| None | 950 | 91.6 | 1847 | 93.4 | 1 | ||
| One or more | 87 | 8.4 | 131 | 6.6 | 1.29 | (0.98 to 1.71) | |
| Missing | 4 | 6 | |||||
| Current pregnancy fetal problem¶ | 0.03 | ||||||
| None | 1018 | 98.2 | 1960 | 99.1 | 1 | ||
| One or more | 19 | 1.8 | 18 | 0.9 | 2.03 | (1.06 to 3.88) | |
| Missing | 4 | 6 | |||||
| Sex of baby | <0.0001 | ||||||
| Male | 630 | 60.8 | 1009 | 50.9 | 1.50 | (1.27 to 1.76) | |
| Female | 406 | 39.2 | 973 | 49.1 | 1 | ||
| Missing | 5 | 2 | |||||
| Gestation at birth (weeks) | 0.20 | ||||||
| 36–37 | 37 | 3.6 | 50 | 2.5 | 1.46 | (0.91 to 2.35) | |
| 38 | 127 | 12.3 | 213 | 10.8 | 1.18 | (0.90 to 1.53) | |
| 39 | 295 | 28.5 | 623 | 31.5 | 0.93 | (0.78 to 1.12) | |
| 40 | 362 | 34.9 | 714 | 36.1 | 1 | ||
| 41–42 | 216 | 20.8 | 380 | 19.2 | 1.12 | (0.94 to 1.34) | |
| Missing | 4 | 4 | |||||
*Area deprivation quintiles created using children in low-income families measure.37
†Essential hypertension; confirmed cardiac disease; thromboembolic disorder; atypical antibodies; hyperthyroidism; diabetes; renal disease; epilepsy.
‡Unexplained stillbirth/neonatal death or previous death related to intrapartum difficulty; previous baby with neonatal encephalopathy; primary PPH requiring treatment/transfusion; shoulder dystocia; previous caesarean section; other.
§GBS; BMI >35 kg/m2; post-term pregnancy; pre-eclampsia/pregnancy-induced hypertension; preterm prelabour rupture of membranes; substance misuse/alcohol; gestational diabetes; other.
¶Malpresentation; small for gestational age; reduced fetal movements; fetal abnormality.
BMI, body mass index; GBS, group B streptococcus; PPH, postpartum haemorrhage.