Healthcare Library

Sexual differentiation

A baby's sex is determined at the time of conception. When the baby is conceived, a chromosome from the sperm cell, either X or Y, fuses with the X chromosome in the egg cell, determining whether the baby will be female or male. Two X's means the baby will be a girl, and XY means it will be a boy. But even though gender is determined at conception, the fetus doesn't develop its external sexual organs until the fourth month of pregnancy. Let's go to seven weeks after conception. You can see from the front that the fetus appears to be sexually indifferent, looking neither like a male or a female. Over the next five weeks, the fetus begins producing hormones that cause its sex organs to grow into either male or female organs. This process is called sexual differentiation. We don't know what sex this fetus is yet, so we'll have to be hypothetical here. Now, if the fetus is a male, it will produce hormones called androgens, which will cause his sexual organs to form like this. On the other hand, a female fetus would not produce androgens; she would produce estrogens' so her sex organs would form like this. Now let's take a look at something you may have missed. At seven weeks, the sex organs of a male and female look identical. Let's add some color to see what happens during sexual differentiation. Keep your eye on the genital tubercle. See that? The genital tubercle formed the penis in the male, (pause) and the clitoris in the female. The penis and clitoris are called sexual analogs (pause) because they originate from the same structure.

Sexual differentiation

Review Date: 5/10/2019

Reviewed By: Linda J. Vorvick, MD, Clinical Associate Professor, Department of Family Medicine, UW Medicine, School of Medicine, University of Washington, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

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