Complications in a first pregnancy, such a pre-eclampsia, were associated with an increased risk of a subsequent preterm birth and relative risks were significantly higher when the complication was recurrent, found a large Australian cohort study in the British Journal of Obstetrics and Gynaecology.
About 27,000 babies are born prematurely, or before 37 weeks' gestation, across Australia each year, with preterm birth the leading cause of death and morbidity in children up to five years of age in the developed world.
Now a study by Curtin University has found that women who give birth to a premature baby after developing pre-eclampsia are 17 times more likely to experience another preterm birth if pre-eclampsia emerges again.
The research examined more than 125,000 women who experienced two consecutive singleton births in Western Australia from 1998 to 2015.
Lead author and PhD candidate Jennifer Dunne, from Curtin’s School of Population Health, said the findings showed the strongest link between preterm birth and pregnancies complicated by pre-eclampsia, a serious pregnancy condition that is usually characterised by high blood pressure, protein in the urine and severe swelling.
“When both pregnancies were complicated by pre-eclampsia, the risk of a subsequent preterm birth increased 10-fold after an initial term birth and 17-fold when the first birth was preterm, compared with women who had an uncomplicated first pregnancy,” Dunne said.
“This study also found that there was a three-fold higher risk of women experiencing a subsequent case of pre-eclampsia after a preterm birth in the first pregnancy that was not complicated by pre-eclampsia.
“Until recently, a first birth at full term was considered a reduced risk for a preterm delivery in the next pregnancy. However, there is emerging evidence that a complicated first pregnancy, regardless of whether the baby was delivered early or at full term, increases the subsequent risk of a baby being born prematurely.”
Dunne said the main pregnancy complications examined included pre-eclampsia, placental abruption (the detachment from the wall of the womb), small-for-gestational age and perinatal death (a stillbirth or a neonatal death in the first 28 days).
“Having any of the four complications in their first pregnancy puts women at an increased risk of a preterm birth in their next pregnancy, regardless of whether that first birth ended at full term or preterm,” she said.
“Likewise, women whose first pregnancy ended in a preterm delivery were at an increased risk for each pregnancy complication in the second pregnancy.
“The findings of this study will help clinicians to better identify women who are at an increased risk of a either a preterm birth or complications in their subsequent pregnancies. Further research is now needed to reveal the specific pathways that explain these strong links between pregnancy complications and preterm births, whether they be genetic, pathological, and behavioural or other recurrent issues.”
The research was supervised by Professor Gavin Pereira and co-authored by Dr Gizachew Tessema, also from Curtin’s School of Population Health.
Study details
The role of confounding in the association between pregnancy complications and subsequent preterm birth: a cohort study
J. Dunne, GA Tessema, G Pereira
Published in BJOG: An International Journal of Obstetrics & Gynaecology on 12 November 2021.
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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