Table 1 Description of cases and controls
Cases (n = 5124)Controls (n = 30053)Unadjusted OR(95% CI)
n (%)*n (%)*
Median (IQR) maternal age at delivery (years)29.6 (25.5–33.2)29.4 (25.5–33.1)1.01 (1.00 to 1.01)
Maternal body mass index (kg/m2)
    Underweight (<18.5)163 (3.2)827 (2.8)1.19 (1.00 to 1.42)
    Normal (18.5–24.9)2031 (39.6)12280 (40.9)Reference
    Overweight (25–29.9)709 (13.8)4131 (13.7)1.04 (0.95 to 1.14)
    Obese (⩾30)385 (7.5)1997 (6.6)1.16 (1.03 to 1.31)
    Missing1836 (35.8)10818 (36.0)1.01 (0.93 to 1.10)
Maternal smoking status
    Non-smoker2590 (50.5)15046 (50.1)Reference
    Ex-smoker269 (5.2)1473 (4.9)1.06 (0.92 to 1.22)
    Current smoker1136 (22.2)6637 (22.1)1.00 (0.92 to 1.08)
    Missing1129 (22.0)6897 (23.0)0.92 (0.84 to 1.01)
Household Townsend score quintile
    1 (least deprivation)826 (16.1)4976 (16.6)Reference
    2592 (11.6)3499 (11.6)1.02 (0.90 to 1.15)
    3586 (11.4)3387 (11.3)1.05 (0.93 to 1.19)
    4506 (9.9)2833 (9.4)1.08 (0.95 to 1.24)
    5 (most deprivation)400 (7.8)2337 (7.8)1.04 (0.90 to 1.21)
    Missing2214 (43.2)13021 (43.3)0.87 (0.67 to 1.12)
Term of pregnancy
    Preterm366 (7.1)858 (2.9)2.62 (2.21 to 3.11)
    Term705 (13.8)4503 (15.0)Reference
    Post-term8 (<0.5)39 (<0.5)1.29 (0.54 to 3.05)
    Missing4045 (78.9)24653 (82.1)1.01 (0.88 to 1.16)
Sex of child
    Female2172 (42.4)14829 (49.4)Reference
    Male2952 (57.6)15224 (50.7)1.32 (1.25 to 1.40)
Singleton or multiple delivery
    Singleton4996 (97.5)29655 (98.7)Matching variable
    Twin128 (2.5)398 (1.3)
Year of birth
    1988–9196 (3.8)1104 (3.7)Matching variable
    1990–41567 (30.6)9192 (30.6)
    1995–91768 (34.5)10389 (34.6)
    2000–41593 (31.1)9368 (31.2)
  • *Proportion of cases or controls. IQR, interquartile range.