680 Excess risk of preterm delivery in monochorionic diamniotic twins: a review of 1536 twin pregnancies


      Twin pregnancies are associated with an excess risk of preterm birth compared to singletons. Monochorionicity further confers an excess risk of pregnancy complications, often related to placental vascular connections and unequal placental sharing. However, the excess risk of prematurity related to monochorionicity has not been well studied in a systematic fashion.

      Study Design

      This study is a retrospective cohort analysis of 23 0/7 - 42 6/7 week twin pregnancies from April 2016 through September 2019 utilizing clinical data from the Obstetrical Care Outcomes Assessment Program (OB COAP). OB COAP is a collaborative perinatal quality improvement initiative based in WA state. The 17 participating sites span all levels of care in urban, suburban and rural settings and provide population-based data including approximately 1/3 of all births in WA state.


      Data were available on 93,780 pregnancies, including 1536 twin pregnancies. Of the 1536 twin pregnancies, 1115 (72.6%) were dichorionic diamniotic (DIDI), 362 (23.5%) were monochorionic diamniotic (MODI), 22 (1.4%) were monoamniotic (MOMO), 37 (2.4%) did not have data on chorionicity. Data on patient characteristics are shown in Table 1. The rates of pre-eclampsia and gestational diabetes were similar between MODI and DIDI twins. However, the risk of preterm birth (<37 weeks) was significantly higher for MODI than DIDI twin pregnancies (81.5% vs 52.6%), p<0.001. This was largely due to an excess risk of preterm birth between 28 and 34 weeks (relative risk 1.9; 95% CI 1.6-2.2) (Table 2). The rate of cesarean birth, especially pre labor cesarean birth, was significantly higher in MODI vs DIDI (77.9% vs 68.7%; p=0.001).


      MODI twins have a significantly increased risk of prematurity, especially prematurity prior to 34 weeks. Determination of chorionicity early in pregnancy is crucial to allow for close surveillance and appropriate clinical management, as well as referral if needed.
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