Risk of singleton preterm birth after prior twin preterm birth: a cohort study

Objective: To determine whether preterm birth in a twin pregnancy increases the risk of subsequent preterm birth (PTB) in a singleton pregnancy and to identify predictors for singleton PTB in women with previous twin PTB.Method: A retrospective cohort study of women with prior twin birth followed by a singleton birth in two tertiary referral centres between 2000 and 2016 was conducted. Rate of PTB in subsequent singleton pregnancy was compared between women who experienced previous preterm versus term twin birth. Analysis was further stratified by gestational age at twin and singleton birth, etiology for PTB and chorionicity in the twin pregnancy.Results: A total of 378 women met the inclusion criteria, of whom 252 (66.7%) experienced PTB in the index twin pregnancy. The overall rate of PTB in the subsequent singleton pregnancy was 11.6% (44/378). Women with prior twin PTB had a higher rate of PTB in the subsequent singleton pregnancy compared with women with prior term twin birth (17.5 versus 6.3%, p = .003, or 3.12, 95%-CI 1.42-6.85), mainly due to a higher rate of late singleton preterm birth (13.1 versus 4.0%, p = .005). Findings of higher odds of PTB in the subsequent singleton pregnancy was limited to women who gave birth before 34^0/7 weeks in the twin pregnancy and was related to the degree of prematurity (prior twin PTB at 30^0/7-33^6/7weeks: OR 3.09, 95%-CI 1.12-8.51; prior twin PTB at <30^0/7weeks: OR 5.8, 95%-CI 2.46-13.68). The association between previous twin PTB and subsequent singleton PTB was limited to women with prior spontaneous twin PTB (OR 3.34, 95%-CI 1.50-7.45).Conclusion: Women with a history of spontaneous PTB in a twin pregnancy are at increased odds of PTB in subsequent singleton pregnancies compared to women with prior term twin birth, and the risk is related to the severity of prematurity in the index twin pregnancy.