Title: The impact of preterm birth in twin pregnancy on subsequent singleton pregnancy outcomes
Abstract: Preterm birth (PTB) in singleton gestations is a risk factor for PTB in future pregnancies. Twin pregnancies are also at high risk for PTB. It is generally believed that PTB in twin pregnancies does not predict a similar outcome in a subsequent singleton pregnancy. Our goal was to evaluate the impact of adverse outcomes, specifically PTB, spontaneous preterm birth (SPB) and early spontaneous preterm birth (EPB: <34 weeks) in a penultimate twin gestation on similar outcomes in the subsequent singleton pregnancy. We identified all women who delivered consecutively a twin followed by a singleton pregnancy at our institution from 1974-2004. Those with an intervening pregnancy delivering after 20 weeks were excluded. Individual chart review confirmed obstetric history, gestational age of and indications for all PTBs. The occurrence of PTB, SPB and EPB in the twin gestation was correlated to outcomes in the singleton pregnancy. p<0.05 was considered significant. 324 women met inclusion criteria. 61.7% of twin pregnancies delivered preterm vs 17.0% of singleton pregnancies (p<0.0001). PTB in the twin pregnancy increased a woman's risk for PTB in the subsequent singleton pregnancy (23 vs 7.3% for twins at term, RR:3.17 [1.7-5.9], p=0.002). Women with twin SPB were at increased risk for subsequent singleton SPB (22.1% vs 6.9% for twins without SPB, RR: 3.51[1.9-6.6], p=0.0002). The risk of subsequent singleton SPB increased with decreasing gestational age at delivery of the twin pregnancy (24.7 vs 16.4 vs 6.9% for EPB, SPB at 34-36 weeks and no SPB, p=0.0005). The impact of SPB in twin gestations on subsequent singleton pregnancy outcomes persisted after controlling for antecedent term birth or SPB before the twin pregnancy (p<0.02). Contrary to general belief, PTB in a twin pregnancy is associated with an increased risk of preterm birth in a subsequent singleton pregnancy. This correlation is stronger with early preterm twin birth. Prior twin pregnancy outcome should be used in counseling women with subsequent singleton pregnancies.