The association between maternal biomarkers and pathways to preterm birth in twin pregnancies

Objective: We sought to estimate the association between cervical length (CL) and fetal fibronectin (fFN) and each pathway leading to preterm birth in twin pregnancies. Methods: Cohort study of 560 patients with twin pregnancies who underwent routine serial CL and fFN screening from 22 to 32 weeks in one maternal fetal medicine practice during 2005-2013. We calculated the association between a short CL (≤20mm) or positive fFN with overall preterm birth <32 weeks, and then subdivided the analysis into preterm birth <32 weeks from preterm labor, preterm premature rupture of membranes (PPROM) and indicated causes. We excluded cases of monochorionic-monoamniotic placentation, vasa previa, twin-twin transfusion and patients with cerclage. Results: The overall rate of preterm birth <32 weeks was 6.9% (3.9% from preterm labor, 1.6% from PPROM and 1.4% indicated). A short cervix was associated with preterm birth <32 weeks arising from preterm labor (12.4% versus 2.0%, p<0.001), but not PPROM (1.9% versus 1.3%, p=0.651). Positive fFN was associated with preterm birth <32 weeks both from preterm labor (17.0% versus 2.4%, p<0.001) as well as from PPROM (5.7% versus 1.0%, p=0.034). Neither was significantly associated with preterm birth <32 weeks from indicated causes. Conclusions: The mechanism leading toward preterm influences the accuracy of screening tests chosen to assess risk in twin pregnancies. A shortened cervical length and positive fFN is associated with spontaneous preterm labor and birth <32 weeks. However, PPROM does not appear to be preceded by a short cervix, but is preceded by a positive fFN. Neither test is associated with an indicated preterm birth.