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Boy or girl? Learning the sex of your baby is a big moment for parents-to-be. Boy and girl genitals look pretty much the same until around 14 weeks or later, so it can be hard to get an answer via ultrasound before then. The most reliable way to find out your baby's sex earlier is with a blood test or genetic test.
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 and scrotum in boys, the clitoris and labia 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 6 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, are recognizable in his abdomen at 9 weeks. By 12 weeks, the bud between his legs has elongated to form his penis. The scrotum, which will later house the testes, forms from bulges located on either side of the developing penis.
At 14 weeks or later, the testes will begin their descent into the scrotum, but it's a long journey. They won’t reach their final position until late in pregnancy or, for some boys, not until after birth.
A girl's uterus and ovaries are evident around 12 weeks. By 22 weeks, her ovaries have completely formed and moved from her abdomen to her pelvis. Amazingly, they already contain a lifetime supply of around 6 million eggs!
By 14 weeks, the genital bud between a girl baby's legs has become the clitoris, and the bulges on either side form the labia. The outer genital area, called the vulva, is now complete.
Kidneys, bladder, and urethra
As the internal sex organs are developing in the abdomen, your baby's kidneys are forming nearby. As your baby grows, the kidneys will move upward until they reach their final location near the lower part of the back.
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.
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. A girl's urethra opens between the clitoris and vagina, and extends back up to the bladder from there.
The kidneys start making urine by 14 weeks of pregnancy. The bladder and urethra are also formed by then, so your baby is able to pee out the urine into the amniotic fluid. 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!
What you can do during pregnancy
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 midpregnancy ultrasound exam, around 18 to 22 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.
For fun, learn about the characteristics you may have that could tilt your odds toward having a boy or girl, or try our (completely unscientific) Chinese gender-predictor tool.
Key milestones in the development of the genitals, internal sex organs, and urinary system
|6 weeks||External genitals of boys and girls look the same|
|7 weeks||Primitive internal sex organs and kidneys start forming|
|9 weeks||Testes are evident in boys|
|10 weeks||Kidneys hook up to the bladder|
|11-12 weeks||External genitals of boys and girls start to differ; uterus and ovaries are evident in girls|
|14 weeks||External genitals are recognizable as male or female; kidneys are making urine; your baby swallows and pees out amniotic fluid|
|18-22 weeks||Typical timing for midpregnancy ultrasound exam, when your baby's genitals can be seen|