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1273Confounding in the association between pregnancy complications and subsequent preterm birth


 
 
 Underlying causal mechanisms that persist from pregnancy to pregnancy have potential to explain recurrence of pregnancy complications. We aimed to estimate the degree of confounding necessary to explain these associations.
 
 
 
 This was a retrospective cohort study of women (n = 124,936) giving birth to their first and second singleton children in Western Australia 1998-2015, identified from the Midwives’ Notifications System. The investigated pregnancy complications were preterm birth (<37 gestational weeks), pre-eclampsia, placental abruption, small-for-gestational-age and perinatal death. Adjusted relative risks (RR) and confidence intervals (CI) were reported. We simulated maternal obesity and derived E-values, a method to determine the magnitude of unmeasured confounding.
 
 
 
 Complications in first pregnancy were associated with higher risk of preterm birth in second pregnancy. RR’s were significantly higher when the complication was recurrent. For the association between pre-eclampsia at first term birth and subsequent preterm birth, the RR increased from 1.2 (95% CI 1.05-1.41) to 11.9 (95% CI 9.52-11.49) when the complication reoccurred. E-values were 1.73 and 23.22 respectfully. Relative risks did not change after adjustment for maternal obesity.
 
 
 
 The strong associations between pregnancy complications and preterm birth support the assumption of shared underlying causes that persist from pregnancy to pregnancy. High E-values suggest that recurrent confounding is unlikely, as any such unmeasured confounder would have to be uncharacteristically large.
 
 
 
 Well-established single confounders cannot explain away the strong associations between complications in first pregnancy and a subsequent preterm birth.
