Ontogeny of isolated ultrasound markers for fetal aneuploidy

Objective: To evaluate the natural history of isolated ultrasound markers for fetal aneuploidy observed at 14-16 weeks of affected gestations. Study Design: 76 aneuploid gestations were diagnosed among a predominantly low-risk population undergoing targeted ultrasonography in the early second trimester. Indications for evaluation of fetal karyotype included fetal malformations or sonographic markers for aneuploidy, maternal age over 35 years and abnormal serum screening results. Markers were re-evaluated at the time of amniocentesis or at pregnancy termination for fetal anomalies. Results: Sonographic markers for aneuploidy (SMA) were observed in 68 of 76 aneuploid gestations diagnosed in the study group, in 46 cases, SMA were associated with major fetal malformations and in 22 cases, markers were isolated. Only 2 of 22 isolated markers for aneuploidy were documented at the time of amniocentesis. Conclusion: Isolated ultrasound markers for aneuploidy are transient and may disappear later on in gestation. Transvaginal sonography at 14-16 weeks of gestation appears to provide the best time window for detection of markers for fetal abnormal karyotype.