Reliability of transvaginal ultrasonography at 7-9 weeks' gestation in the determination of chorionicity and amnionicity in twin pregnancies.

OBJECTIVES

To assess the agreement in the diagnosis of chorionicity and amnionicity between transvaginal ultrasound examination at 7-9 weeks' gestation and at the 11-14-week transabdominal scan.

METHODS

This was a retrospective analysis of prospectively collected data of twin pregnancies over a 3-year period. Chorionicity and amnionicity were assessed in women with viable twin pregnancies who underwent a transvaginal ultrasound scan at between 7 and 9 weeks' gestation. These findings were compared with the diagnoses of chorionicity and amnionicity at the subsequent 11-14-week scan in these women.

RESULTS

Chorionicity and amnionicity were documented in 67 viable twin pregnancies at both 7-9 and 11-14 weeks' gestation. There was agreement in the chorionicity and amnionicity reported at each of the two scans in 65 out of 67 (97%) cases. Of the dichorionic-diamniotic (DCDA) pregnancies reported at 7-9 weeks, 53 out of 54 (98%) were confirmed at the 11-14-week scan and one (2%) was found to be monochorionic-diamniotic (MCDA). However, at birth these twins were of different sex, confirming DCDA twins as initially diagnosed at 7-9 weeks. Of the 12 pregnancies diagnosed as MCDA at 7-9 weeks, all were found to be MCDA at the 11-14-week scan. There was one monochorionic-monoamniotic (MCMA) pregnancy diagnosed at 7-9 weeks that was subsequently found to be MCDA at the 11-14-week scan.

CONCLUSION

Transvaginal ultrasound examination at 7-9 weeks' gestation shows very high agreement with the 11-14-week scan in the diagnosis of chorionicity and amnionicity in twin pregnancies, suggesting that it provides a similar level of accuracy. Accuracy may be higher for DC twins than MC twins, which may relate to the gestational age at which the sonographic appearance of the amniotic sac develops.