FETAL SEX DETERMINATION IN THE FIRST TRIMESTER

EMBRYOLOGY

Link to Embryology

INDICATIONS FOR FETAL SEX DETERMINATION

Link to Indications for Fetal Sex Determination

ULTRASOUND

• Most workers agree that sagittal images are more helpful than transverse images (1-3).
• Transverse images:
• Male = Uniform genital swellings form a dome shaped structure (scrotum) at base of fetus. Longitudinal midline echo at base of the penis is the median penile raphe.
• Female = Genital swellings form 2 or 4 parallel lines (labia minora and majora).
• No appreciable difference in the size of the penis and clitoris until after 14 weeks gestational age.
• Assessment of the direction in which the genital tubercle points:
• Male phallus is vertically - cranially directed as the corpora cavernosa of the penis are permanently congested and the penis is in a constant erect state.
• Female tubercle is caudally directed.

	Femal tubercle
	Caudal notch

• "Sagittal sign" - midline sagittal view of the genital area demonstrates a caudal notch in the female and a cranial notch in the male (2,4).
• Angle of the genital tubercle (1):
• Mid-sagittal plane with fetus supine and parallel to probe.
• No extension of limbs or spine.
• Angle of genital tubercle to a horizontal line drawn through the lumbosacral skin surface.
• Male - angle > 30°.
• Female - phallus parallel or convergent to the horizontal line. -20° to +20° (never greater than 30°).
• Accuracy = 80% at 12-14 weeks and 90% after 14 weeks (1).

Male Fetus at 12 weeks GA – note acute angle of the penis
Female Fetus at 14 weeks GA – note the converging angle of the clitoris

REFERENCES

1. Whitlow BJ, Lazanakis MS, Economides DL. The sonographic identification of fetal gender from 11-14 weeks of gestation. Ultrasound Obstet Gynecol 1999;13:301-304.
2. Efrat Z, Akinfenwa OO, Nicolaides KH. First-trimester determination of fetal gender by ultrasound. Ultrasound Obstet Gynecol 1999;13:305-307.
3. Mazza V, Contu G, Falcinelli C et.al. Biometrical threshold of biparietal diameter for certain fetal sex assignment by ultrasound. Ultrasound Obstet Gynecol 1999;13:308-311.
4. Emerson DS, Felker RE, Brown DL. The sagittal sign. An early second trimester sonographic indicator of fetal gender. J Ultrasound Med 1989;8:293-297.