External genitalia of both
sexes are similar and may be impossible to differentiate between.
Genital swellings form on
either side of the genital tubercle and urethral fold.
WEEKS 6-12
In the male the genital
tubercle elongates and the urethral folds close over the urethral plate to
form the penile urethra. The ureteral tubercle is known as the phallus.
The genital swellings form
the scrotum (may be confused with the labia majora and minora which also
develop from the genital swellings).
WEEKS 12-13
The process of
gender-specific changes occur:
In the male the urogenital
sinus is replaced by the scrotum and urethral raphe. Closure of the
urogenital sinus occurs in a zip like fashion beginning caudally in the
embryo. At the same time the genital tubercle elongates and displaces the
phallus in a rostral direction.
In the female the urogenital
sinus remains open and becomes the vestibule of the vagina.
WEEK 14
There are significantly
different rates of growth of the penis and clitoris. The penis shows
linear growth from 14 weeks of gestation onwards.
REFERENCES
Efrat Z, Akinfenwa OO,
Nicolaides KH. First-trimester determination of fetal gender by
ultrasound. Ultrasound Obstet Gynecol 1999;13:305-307.
Marshall FF. Embryology of
the lower genitourinary tract. Urol Clin North Am 1978;5:3-15.