Association between change in cervical length and spontaneous preterm birth in twin pregnancies.

Background: There is a lack of consensus on the optimal transvaginal cervical length for determining risk for spontaneous preterm birth in twin pregnancies. Change in transvaginal cervical length over time may reflect early activation of the parturition process, as has been demonstrated in singleton pregnancies. The association between change in transvaginal cervical length and the risk for spontaneous preterm birth has not yet been described in the population of women with diamniotic twin pregnancies.

Objective: Our primary objective is to determine whether rate of change in transvaginal cervical length in the midtrimester is associated with spontaneous preterm birth in twin gestations. Our secondary objective is to describe parameters for identifying patients at increased risk for spontaneous preterm birth based on change in transvaginal cervical length over time.

Study Design: This is a retrospective cohort of serial transvaginal cervical length performed for twin pregnancies at a single institution from 2008 through 2015. Women with diamniotic twin pregnancies who had transvaginal cervical length measurements at 18 and 22 weeks' gestation and outcome data available were included. Logistic regression was used to determine the relationship between the rate of change in transvaginal cervical length and the risk for the primary outcome of spontaneous preterm birth <35 weeks as well as spontaneous preterm birth <32 weeks.

Results: In all, 527 subjects met inclusion criteria for this study. The average rate of change in transvaginal cervical length for patients with spontaneous preterm birth <35 weeks was -0.21 cm/wk (SD 0.27) vs -0.10 cm/wk (SD 0.24) for patients who delivered ≥35 weeks (P < .01). The rate of change in transvaginal cervical length was associated with spontaneous preterm birth <35 weeks when controlling for initial transvaginal cervical length and other important risk factors for spontaneous preterm birth. Results for spontaneous preterm birth <32 weeks were similar. This association remained significant when the rate of weekly change was treated as a dichotomous variable based on an apparent inflection point in the risk for spontaneous preterm birth: women with rapid change in transvaginal cervical length, ≥-0.2 cm/wk, had 3.45 times the odds of spontaneous preterm birth as those with less rapid change (95% confidence interval, 2.15-5.52) when controlling for initial transvaginal cervical length.

Conclusion: Change in transvaginal cervical length in the midtrimester is associated with spontaneous preterm birth, and therefore protocols for serial transvaginal cervical length measurement can provide the clinician with information to identify at-risk patients. A decrease of ≥0.2 cm/wk of transvaginal cervical length identifies patients at increased risk for spontaneous preterm birth <35 weeks.