Male vs. Female Development

A baby’s sex is determined at conception by the combination of X and Y chromosomes passed on from the mother’s egg and the father’s sperm. Males have a set of XY sex chromosomes and females have a set of XX sex chromosomes. During pregnancy, a woman’s bloodstream contains not only her own DNA, but also trace amounts of DNA from her unborn child. Non-invasive prenatal DNA testing can determine as early as 6 weeks whether a baby is male or female before being able to detect anatomical differences by ultrasound. If male chromosomes are detected, the baby is a boy; if none are found, the baby is a girl.¹

In females, the lack of AMH causes the gonads to develop into ovaries. The female structures also consist of a pair of tubes called the paramesonephric ducts. Parts of the paramesonephric ducts will later fuse at the midline to form the uterus, cervix and upper portions of the vagina, while the unfused sections become the fallopian tubes. In the absence of testosterone and AMH, the primitive female anatomy matures, and the male structures that require testosterone to survive start to shrivel. Therefore, the presence of the Y chromosome in males and absence in females, is the fundamental mechanism establishing human sex and reproductive development.³

Externally, both male and female embryos have indistinguishable genitalia for the first eleven weeks gestation. Both sexes develop folds of tissue adjacent to the external openings that lead to the bladder and the intestines. Both sexes also have a small protrusion of tissue known as the genital tubercle.⁴ Because of this, early ultrasounds cannot typically determine whether the baby is a boy or a girl.  By 11 to 14 weeks, ultrasound can often identify the baby’s sex, with accuracy improving from about 75% to almost 100% by week 14.^{5 6}

Around 11 weeks gestation, the external structures start to differentiate. In males, the genital tubercle elongates and becomes the penis. The external folds fuse at the midline and become the scrotum. By 14 weeks, the male genitalia are mostly formed.⁷ In females, the genital tubercle elongates only slightly, forming the clitoris, and the surrounding folds become the labia. Female external genitalia form between 13 and 22 weeks.⁸