Department of Obstetrics and Gynecology, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok 10700, Thailand.
Objective: To assess the sonographic morphology of umbilical cord coiling in chromosomally normal fetuses and estimate the specificity of hypocoiled appearance as a marker for fetal trisomy 21 in the second trimester. Methods: 555 singleton pregnancies undergoing amniocentesis for fetal karyotype were scanned at 17-22 weeks’ gestation. The umbilical cord was evaluated in a longitudinal section using 2D with color Doppler sonography. The cross-sectional data of umbilical coiling index (UCI), defined as the reciprocal of the distance between umbilical coils, were analyzed to establish the normal range according to the gestational age. A UCI less than the 5th percentile was the cut-off value for hypocolied umbilical cord. The specificity of hypocolied umbilical cord to predict fetal trisomy 21 was calculated. Results: As a result of amniocentesis, there were 527 chromosomally normal fetuses and 9 trisomy 21 fetuses. There was no significant difference in maternal characteristics between these two groups. In trisomy 21 fetuses, there was a higher proportion of fetal anomaly indicating for fetal karyotype (22.2% VS 0.6%, p = 0.003). The regression equation of UCI (cm-1, y) on gestational age (weeks, x) was y = 1.205 - 0.033x. The hypocoiled umbilical cord was characterized in 2/9 (22.2%) fetuses with trisomy 21 and in 22/527 (4.2%) chromosomally normal fetuses (p = 0.01), with specificity of 95.83%. Conclusion: The coiling pattern of umbilical cord visualized by sonography has a potential value in second-trimester screening for fetal trisomy 21. Nevertheless, further studies of this model in a larger cohort would provide more information in sensitivity and predictive values.