The role of confounding in the association between pregnancy complications and subsequent preterm birth: a cohort study
Date
2022
Authors
Dunne, J.
Tessema, G.A.
Pereira, G.
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Journal article
Citation
BJOG: An International Journal of Obstetrics and Gynaecology, 2022; 129(6):890-899
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J Dunne, GA Tessema, G Pereira
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Abstract
Objective: To estimate the degree of confounding necessary to explain the associations between complications in a first pregnancy and the subsequent risk of preterm birth. Design: Population-based cohort study.
Setting: Western Australia. Population: Women (n = 125 473) who gave birth to their first and second singleton children between 1998 and 2015.
Main outcome measures: Relative risk (RR) of a subsequent preterm birth (<37 weeks of gestation) with complications of pre-eclampsia, placental abruption, small-for-gestational age and perinatal death (stillbirth and neonatal death within 28 days of birth). We derived e-values to determine the minimum strength of association for an unmeasured confounding factor to explain away an observed association. Results: Complications in a first pregnancy were associated with an increased risk of a subsequent preterm birth. Relative risks were significantly higher when the complication was recurrent, with the exception of first-term perinatal death. The association with subsequent preterm birth was strongest when pre-eclampsia was recurrent. The risk of subsequent preterm birth with pre-eclampsia was 11.87 (95% CI 9.52–14.79) times higher after a first term birth with pre-eclampsia, and 64.04 (95% CI 53.58–76.55) times higher after a preterm first birth with pre-eclampsia, than an uncomplicated term birth. The e-values were 23.22 and 127.58, respectively. Conclusions: The strong associations between recurrent pre-eclampsia, placental abruption and small-for-gestational age with preterm birth supports the hypothesis of shared underlying causes that persist from pregnancy to pregnancy. High e-values suggest that recurrent confounding is unlikely, as any such unmeasured confounding factor would have to be uncharacteristically large.
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Published Online 30 November 2021
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© 2021 John Wiley & Sons Ltd.