We sought to determine an optimal gestational-age cutoff of preterm twin deliveries for predicting subsequent singleton preterm birth (PTB).
We performed a retrospective study of women with a spontaneous twin delivery who subsequently had a singleton gestation. Univariate and multivariate analyses determined the risk of a spontaneous singleton PTB after a PTB of a twin gestation. Different gestational-age cutoffs of the previous twin PTB were evaluated.
Among 255 women, previous twin PTB at <34 weeks’ gestation was associated with an increased risk of singleton PTB (odds ratio, 9.67; 95% confidence interval, 3.07–30.47). Every twin gestational age cutoff at <34 weeks’ gestation had a significantly higher risk of subsequent singleton PTB, which was no longer significant at ≥34 weeks’ gestation (odds ratio, 1.68; 95% confidence interval, 0.23–12.19).
In women with a previous spontaneous twin PTB at <34 weeks’ gestation, there is an increased risk of subsequent singleton PTB. A twin birth at ≥34 weeks’ gestation is not associated with an increased risk for a subsequent singleton PTB.
Preterm birth (PTB) remains at the forefront of obstetric challenges today. The PTB rate remains >12% in the United States, with well over 500,000 infants per year being born prematurely. Prematurity remains the leading cause of infant morbidity and death. The challenge is 2-fold: (1) identification of those populations at higher risk for PTB and (2) provision of interventions that prevent PTB. It is well accepted that one of the greatest risk factors in the prediction of spontaneous singleton PTB (sPTB) is previous singleton PTB. Subsequent to the identification of this at-risk population, various secondary preventative measures (eg, progesterone, ultrasound-indicated cerclage) have been shown to reduce the risk of subsequent sPTB.
The risk of delivering a singleton infant prematurely after a preterm delivery of a twin gestation, however, is not clear. Although 2 studies have shown no increased risk of a subsequent sPTB after a twin PTB, 2 other studies have shown an increased risk. An additional question exists, in that the definition of PTB in twin infants (<37 weeks’ gestation) is defined within the same parameters as that of a singleton infant, despite the fact that 60% of twins are born before this gestational age (GA). What makes these twin pregnancies deliver prematurely is not known: is a PTB the result of simply carrying a twin gestation, or does a woman with a twin PTB at an earlier GA also possess an inherent increased risk of having a PTB, which carries on to a future pregnancy? It was therefore our intention to evaluate whether a previous PTB of a twin gestation is associated with an increased risk of a subsequent singleton PTB and, if yes, to find an optimal twin GA cutoff in predicting this risk.
Materials and Methods
We performed an institutional review board–approved retrospective cohort study that analyzed a group of women with a twin delivery who subsequently had a singleton delivery from 1996-2010. These women delivered both of their pregnancies at Christiana Care Health Systems, Newark, DE. Data were extracted from a preexisting database that contains various demographic and delivery information. This database contains selected obstetric and neonatal outcomes for all women who deliver at this institution. The data are entered by nurses who attend the deliveries; quarterly audits are used to ensure accuracy of data entry. These audits consistently have verified the accuracy of the data to exceed 95%, with additional independent validation that demonstrates the accuracy of key variables.
Subjects were considered for inclusion if the following 2 criteria were met: singleton gestation and previous twin gestation. Subjects were excluded for indicated (iatrogenic) PTBs at <37 weeks’ gestation in either pregnancy, previous PTB preceding the twin delivery, intrauterine death of 1 or both twins, major fetal anomaly, or unclear or incomplete delivery information. The “preterm twin” group consisted of women who had a previous twin PTB between 18 and 36 weeks 6 days’ gestation (after spontaneous preterm labor or preterm premature rupture of membranes). The “term twin” group consisted of women who had a twin birth at ≥37 weeks’ gestation.
Demographic information that we extracted included maternal age, race, body mass index (kilograms per square meter), smoking history, insurance status, parity, and the interpregnancy interval from the twin delivery to the conception of the singleton gestation. The initial query specified exclusion of any woman who had a PTB preceding the twin pregnancy; chart-level documents were reviewed where appropriate if there was a question regarding spontaneous vs iatrogenic delivery.
Statistical analysis was performed using SPSS-PC software (version 16.0; SPSS Inc, Chicago, IL). Categoric variables were compared with the use of χ 2 tests or Fisher’s exact test, as appropriate. Continuous variables were compared with the use of the Student t test or the Mann-Whitney U test for nonnormally distributed variables. Logistic regression was used to assess the impact of potential confounders. A probability value < .05 was considered significant. When appropriate, odds ratios (ORs) with 95% confidence intervals (CIs) were estimated. A receiver operator curve was used to identify which GAs of a previous twin delivery were associated with an increased risk of spontaneous PTB in a subsequent singleton gestation, which was defined as <37 weeks’ gestation.
There were 308 women who were identified as having had a twin delivery followed by a singleton pregnancy in the absence of a PTB preceding the twin gestation. A total of 53 women were excluded after chart-level review ( Figure 1 ) . After exclusion for the aforementioned criteria, there were 255 women whose data were available for analysis. Of these, 144 women (56.5%) had a history of a twin PTB, and 111 (43.5%) had a twin term birth. Among the variables that were studied, there were no demographic differences between the preterm twin group and term twin group at the time of the singleton pregnancy ( Table 1 ).