Patterns of recurrent preterm birth in Western Australia: A 36-year state-wide population-based study.

Background: It is known that a previous preterm birth increases the risk of a subsequent preterm birth, but a limited number of studies have examined this beyond two consecutive pregnancies.

Aims: This study aimed to assess the risk and patterns of (recurrent) preterm birth up to the fourth pregnancy.

Materials and methods: We used Western Australian routinely linked population health datasets to identify women who had two or more consecutive singleton births (≥20 weeks gestation) from 1980 to 2015. A log-binomial model was used to calculate risk ratios (RRs) and 95% confidence interval (CIs) for preterm birth risk in the third and fourth deliveries by the combined outcomes of previous pregnancies.

Results: We analysed 255 435 women with 651 726 births. About 7% of women had a preterm birth in the first delivery, and the rate of continuous preterm birth recurrence was 22.9% (second), 44.9% (third) and 58.5% (fourth) deliveries. The risk of preterm birth at the third delivery was highest for women with two prior indicated preterm births (RR 12.5, 95% CI: 11.3, 13.9) and for those whose first pregnancy was 32-36 weeks gestation, and second pregnancy was less than 32 weeks gestation (RR 11.8, 95% CI: 10.3, 13.5). There were similar findings for the second and fourth deliveries.

Conclusions: Our findings demonstrate that women with any prior preterm birth were at greater risk of preterm birth in subsequent pregnancies compared with women with only term births, and the risk increased with shorter gestational length, and the number of previous preterm deliveries, especially sequential ones.