Prenatal detection of fetal aneuploidy by sonographic ear length.

Objective: To determine the usefulness of a fetal ear length nomogram in the prenatal detection of fetal aneuploidy and to determine whether ear smallness in cases of aneuploidy is a primary or secondary event.

Methods: Ear lengths of 447 singleton fetuses (October 1996 to October 1997)were prospectively evaluated between 14 and 41 weeks to establish a nomogram created by modeling the mean and SD separately. Records of aneuploid fetuses were retrospectively reviewed, and their ear lengths were plotted against the nomogram to determine detection rates, with ear length in or below the 10th and 50th percentiles for a given gestational age and biparietal diameter used as abnormal cutoffs.

Results: The nomogram for fetal ear length measurements provided sufficient data to derive the 10th, 50th, and 90th percentiles on the basis of gestational age and biparietal diameter. The ear length of euploid fetuses was significantly correlated with gestational age (R2 = 0.96; P < .001) and biparietal diameter (R2 = 0.95; P < .001). From 96 aneuploid fetuses identified, 63 had ear lengths in or below the 10th percentile for gestational age (sensitivity, 66%). When using ear length against biparietal diameter, the sensitivities for all aneuploid fetuses for cutoffs at or below the 10th and 50th percentiles were 43% (40 of 93) and 83% (77 of 93), respectively.

Conclusions: Most aneuploid fetuses have sonographically small ears (< or = 10th percentile for gestational age). This smallness is not entirely related to overall small fetal size, but in almost half the cases, the fetal ear length is disproportionately smaller than the biparietal diameter.