DETERMINATION IN THE FIRST TRIMESTER
Link to Embryology
INDICATIONS FOR FETAL SEX DETERMINATION
Link to Indications for Fetal
- 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
- "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
- Mid-sagittal plane
with fetus supine and parallel to probe.
- No extension of limbs
- 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
at 14 weeks GA – note the converging angle of the clitoris
- 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.
- Efrat Z, Akinfenwa OO, Nicolaides
KH. First-trimester determination of fetal gender by ultrasound.
Ultrasound Obstet Gynecol 1999;13:305-307.
- 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.
- 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.