How Human Reproduction Works

Many expectant parents look forward to the doctor's appointment that comes 20 weeks into the pregnancy -- that's when an ultrasound can usually reveal the sex of their baby. The sex of that baby is determined at the exact moment when egg and sperm meet, but it takes a while for the external organs to match the internal chromosomes.

As an embryo develops, it acquires both Wolffian and Mullerian ducts. Wolffian ducts develop into male sex organs, and Mullerian ducts develop into female sex organs. Which sex organs develop depends on the presence of a Y chromosome and the male hormone testosterone and anti-Mullerian hormone (AMH). At eight weeks, the internal genitalia will begin to form. If the embryo has both an X and a Y chromosome and produces the two hormones, then the testosterone will stimulate the Wolffian duct to develop male sex organs, including the vas deferens and the seminal vesicles. If there's no Y chromosome, but two X chromosomes instead, then the embryo is female. The Wolffian duct will degrade, and the Mullerian duct will develop into female sex organs such as the uterus, fallopian tubes and part of the vagina. Rarely, the embryo will have an X and a Y chromosome, but will fail to produce testosterone or AMH; such an embryo is termed intersex, as it has both male and female sex organs.

The external genitals continue to develop after the internal ones have formed. Testosterone produces a penis and a scrotum for the male embryo, while the lack of testosterone will lead to a clitoris, urethra, the rest of the vagina and the labia for the female. These organs will continue to develop during pregnancy, and then they will undergo another period of development at puberty. On the next page, we'll take a closer look at the functions these organs perform.