Open-legs axial plane: A standardized methodology and reference values for fetal genital biometry in mid-trimester ultrasound

Abstract

Background: Evaluation of the external genitalia is an important part of prenatal ultrasound. However, there is no standardized methodology that includes biometric measurements and normative data to be able to carry out this evaluation.

Objective: To develop a standardized methodology for fetal genital biometry and obtain reference values for use in mid-trimester ultrasound.

Study design: A prospective cross-sectional study was used. 273 male and 253 female fetuses of normal, singleton pregnancies at 18 to 22 weeks were included. Measurements of fetal penis length, penile width and transverse scrotal diameter in male fetuses and bilabial diameter in female fetuses were performed by transabdominal ultrasound. Reference values were calculated for each gestational week.

Results: Realization of the open-legs axial plane is described as a working methodology. Normative data for penile length, penile width, transverse scrotal diameter and bilabial diameter are defined, including mean, minimum and maximum values, range, and 5th, 10th, 90th and 95th percentiles.

Conclusions: We have provided a standardized methodology using the open-legs axial plane, which would integrate the main measurements. In addition with the normative data constructed from their use, we hope to be able to improve the external genitalia assessment and diagnosis of genital anomalies in mid-trimester ultrasound.

Keywords: Bilabial diameter; External genitalia; Female sexual development; Fetal penile length; Fetal scrotum; Genital abnormalities; Male sexual development; Micropenis.