Sexual differentiation

<< View all animations

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.


Need an appointment? Request one online 24 hours/day, 7 days/week or call 800-789-PENN (7366) to speak to a referral counselor.


A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission ( URAC's accreditation program is the first of its kind, requiring compliance with 53 standards of quality and accountability, verified by independent audit. A.D.A.M. is among the first to achieve this important distinction for online health information and services. Learn more about A.D.A.M.'s editorial process. A.D.A.M. is also a founding member of Hi-Ethics ( and subscribes to the principles of the Health on the Net Foundation (

©Copyright A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.

The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed physician should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites.

Related Links
Request an Appointment Online or call
1-800-789-PENN (7366)




About UPHS   Contact Us   Site Map   Privacy Statement   Legal Disclaimer   Terms of Use

The University of Pennsylvania Health System, Philadelphia, PA 1-800-789-PENN © 2016, The Trustees of the University of Pennsylvania