Transvaginal sonography (TVS) enables sex determination at an early stage of pregnancy. The morphologic features of fetal external genitalia at 13 weeks to 16 weeks, menstrual age, are different from those seen later in pregnancy; therefore an attempt to determine fetal gender at this early stage by the same criteria as those used later is hazardous, especially for determining the male sex. The main diagnostic criteria for male gender determination by TVS are the “dome” sign representing the sonographic visualization of the fetal scrotum, the cranially directed phallus, and the longitudinal raphe at the base of the penis. The diagnostic criteria for female gender are the 2 or 4 parallel lines representing the labial folds and the caudally directed phallus (clitoris). The length of the fetal phallus at this early stage is not diagnostic and may be the main pitfall to the unexperienced sonographer.
Between weeks 13 and 14 sex diagnosis was possible in 130/171 pregnancies (76%) in our first 2 years and 1881235 (80%) in our last 2 years of experience. Between weeks 15 and 16 sex diagnosis was possible in 122/139 pregnancies (88%) during our first 2 years and 96.7% (528/546) during the last 2 years of experience. The accuracy rate for fetal male gender identification increased from 91.7% during the first 2 years of TVS experience to 99.7% during the last 2 years of TVS experience, and the accuracy rate for female gender identification, increased from 93.3% to 100%, respectively, applying the above criteria and based on acquired experience of early fetal sex identification by TVS early in gestation. Early and precise determination of fetal sex is possible and might avoid invasive procedures such as amniocentesis.