Objectives: A longitudinal evaluation by sonography of external genitalia in human embryos/early fetuses with a known time from fertilization is lacking. Our aim was to assign by sonographic evaluation of external genitalia the early fetal gender in a cohort of pregnancies after in vitro fertilization. Sonographic examinations were performed in each case in three subsequent sessions over a period of time early in gestation in order to establish a temporal threshold, expressed in terms of days from fertilization, at which absolute accuracy in gender prediction is achievable.
Methods: Thirty-two fetuses were included in this prospective longitudinal study. Each was examined three times for gender assignment. The first observation was performed between 65 and 69 days from fertilization, the second between 70 and 74 days and the third between 75 and 79 days. Transvaginal and/or transabdominal sonography was used to detect the 'sagittal sign' as a marker of fetal gender. The results of ultrasound examinations were compared with gender at birth or with karyotype obtained from amniotic fluid cells or chorionic villus sampling.
Results: Fetal gender assignment was feasible in 29 out of 32 fetuses (90%) at the first examination and in all cases at the second and third examinations. Fetal gender prediction was correct in 76% of cases in which fetal gender was assigned (22/29) at the first examination; accuracy for males was 46% (6/13) and for females 100% (16/16). At the second and third examinations, accuracy for gender prediction achieved 100% for both genders. Concerning the temporal threshold, absolute accuracy in gender prediction was achieved at 69 days from fertilization, corresponding to 11+6 weeks based on the last menstrual period.
Conclusion: This study provides important information about the earliest stage, expressed in terms of days from fertilization, at which it is possible to make a certain diagnosis of fetal gender by sonography.