Comparative longitudinal study of cervical length and induced shortening changes among singleton, twin, and triplet pregnancies

Objective: To compare cervical length and induced shortening changes during gestation among singleton, twin, and triplet pregnancies. Study design: Thirty-two healthy gravidas (12 singleton, 13 twin, and 7 triplet pregnancies) between 17 and 20 weeks' gestation were prospectively enrolled in this longitudinal investigation of cervical length. Serial transperineal cervical length ultrasound assessments were made weekly until 34 weeks' gestation under 3 conditions: 1) supine, 2) supine with the Valsalva maneuver, and 3) standing. Cervical length, internal os diameter, and presence of cervical funneling were assessed under each condition. Multiple regression models were created using generalized estimating equations to predict these measures and accounting for confounding effects from covariates and adjusting for correlations from repeated measurements on each woman. Results: A total of 1286 cervical sonographic measurements were made. In a multiple linear regression generalized estimating equations model, estimated cervical length was significantly different among singleton, twin, and triplet pregnancies. Overall, changing maternal position from supine to standing resulted in a nonsignificant change (-0.1 ± 0.4 mm) in cervical length (P = .85). In contrast, measurement of cervical length during the Valsalva maneuver resulted in a significant reduction in cervical length when compared with the cervical length measured in supine (-1.0 ± 0.3 mm) (P = .0009) and standing positions (-0.9 ± 0.4) (P =.009). The observed induced shortening changes were similar across gestation, irrespective of singleton, twin, or triplet gestation. Conclusion: Longitudinal cervical length changes differ significantly throughout gestation among singleton, twin, and triplet pregnancies. Cervical length measurements made in a standing position are comparable with those measured while supine, whereas cervical lengths measured during the Valsalvamaneuver are significantly shorter than those made in either the supine or standing position.