Sonographic biometrical range of external genitalia differentiation in the first trimester of pregnancy: analysis of 2593 cases.

The aim of this study was to establish the accuracy of fetal gender assignment by sonography in the biometrical range of 18 to 29 mm of biparietal diameter (BPD).Transvaginal and/or transabdominal sonography was used to detect the sagittal sign as a marker of fetal gender in 2593 fetuses with BPD between 18 and 29 mm. The results of sonographic examination were compared with the gender at birth or with karyotype obtained from amniotic fluid cells or chorionic villus sampling.Fetal gender assignment was feasible in 2374 of 2593 cases (91\%). Of the 2188 fetuses with known fetal sex outcome, 1025 were males and 1157 were females, and 6 had genital anomalies. In fetuses without genital anomalies, an accuracy rate of 100\% was achieved at a BPD of >/=24 mm. The results of the six cases with genital malformations were considered separately.Sonography is a reliable method for the study of the morphological development of the external genitalia in fetuses 'in vivo'; it is possible to assign fetal gender in 95 to 99\% starting at a BPD of 20 mm and to achieve an accuracy rate of 99 to 100\% from a BPD of 22 mm, but fetal sex assignment should not be undertaken below a BPD of 22 mm, and especially not in cases where fetal sexing affects pregnancy management.