When do your baby's sex organs develop in the womb?

Jul 18, 2023

Your baby’s sex organs and urinary system begin to form very early in pregnancy. Here's how they develop.

Your baby's sex is set at conception by the sex chromosome received from the sperm (X for girl or Y for boy), but it takes a while for the actual body parts to develop.

The internal sex organs – the testes in boys and the uterus and ovaries in girls – look the same until 9 weeks of pregnancy. The external sex organs – the penis in boys, the clitoris and labia majora in girls – don't start to differ from each other until about 11 weeks. And even then, it takes several more weeks to be able to easily see the difference between boys and girls on an ultrasound.

The external genitals start out as several small bulges that form between your baby's legs at 4 to 5 weeks of pregnancy. Around the same time, the internal sex organs are forming from a ridge of tissue on each side of your baby's abdomen. These ridges also give rise to the kidneys, which filter wastes from the blood and produce urine.

A boy's testes, the glands that make and store sperm, begin to develop after 6 to 7 weeks gestation. At 10 weeks, the bud between his legs has elongated to form his penis. It continues to grow in length throughout pregnancy to about 3.5 centimeters at birth. The scrotum, which will later house the testes, forms from bulges located on either side of the developing penis. And at 12 weeks, baby’s prostate appears.

By 20 weeks, the development of the male external genitalia is complete. The testes will begin their descent into the scrotum, but they won’t reach their final position until late in pregnancy, at about 25 to 35 weeks. For some boys, this doesn’t happen until after birth. (Premature and low birth weight infants are more likely to have undescended testes.)

At 8 weeks of pregnancy, external female and male genitalia are still identical. After this time, the fleshy coverings in the genital area either form the labia major, or the scrotum and penis.

A girl's ovarian follicles begin to form around 12 weeks. By 13 weeks, her ovaries are fully developed inside her body. Also at 13 weeks, the genital bud between a baby girl's legs has become the clitoris. At 20 weeks, her reproductive system has fully developed. Amazingly, her ovaries already contain a lifetime supply of 6 to 7 million eggs!

While the internal sex organs are forming in the abdomen during the first trimester, your baby's urinary system is developing nearby. The kidney structures begin developing at six weeks, near the structures that will become the reproductive organs. As your baby grows, the kidneys will move upward until they reach their final location near the lower part of the back by about 12 weeks.

The bladder begins to develop during weeks 9 and 10, and by about 10 weeks of pregnancy, two tubes connect the kidneys to the bladder. As urine is formed in the kidneys, it will flow through these tubes to be stored in the bladder until it's peed out. You might be able to see the bladder on an ultrasound at 10 to 14 weeks, and even see the bladder emptying at 15 weeks.

In a male fetus, the urethra – the tube that allows urine to flow out of the body from the bladder – opens at the tip of the penis. This tube extends up through the penis until it reaches the developing bladder. In a female fetus, the urethra opens between the clitoris and vagina, and extends up to the bladder from there.

The kidneys start making urine by 13 weeks, and this will continue throughout your pregnancy. The bladder and urethra are also formed by then, so your baby is able to pee out the urine into the amniotic fluid. After 20 weeks of pregnancy, the amniotic fluid surrounding your baby is mostly comprised of their urine.

Toward the end of pregnancy, your baby swallows enough amniotic fluid to pee out about 32 ounces of fluid a day – the amount in two standard water bottles. This is important for normal development in the womb, as well as good practice for drinking breast milk or formula after birth.

If you want to know your baby's sex before birth, there are a few ways to find out. The most common is during your anatomy ultrasound exam, around 20 weeks of pregnancy. Keep in mind, though, that it's not always 100 percent accurate, in part because your baby might not be in a good position for the sonographer to see the genitals.

Other methods of determining the sex include noninvasive prenatal testing (NIPT), amniocentesis, and chorionic villus sampling (CVS). All of these are much more accurate than ultrasound because they're based on an analysis of fetal DNA.

Eating well and taking prenatal vitamins are two of the best things you can do during pregnancy to help your baby grow and develop. Folic acid, also known as folate, is an important vitamin that helps prevent major birth defects and contributes to your baby's overall health. You need about 400 to 600 micrograms of folic acid a day during pregnancy, which can come from both food — think fortified cereal and dark green leafy vegetables — and your prenatal vitamin.

Drink plenty of water every day, too. You can always talk to your healthcare provider about ways to support your baby's needs as they grow during pregnancy.

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Maggie Getz is a freelance writer and editor specializing in health, wellness, and motherhood. She lives in Colorado with her husband and young son and daughter. She enjoys hiking, yoga, baking (and eating said baked goods), as well as connecting with other moms.