Advertisement

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

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

pregnant woman holding ultrasound
Photo credit: Thinkstock © iStock

 

Your baby’s sex organ development

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.

Advertisement | page continues below

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.

Male sex organ development

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.)

Female sex organ development

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!

Your baby's urinary system development

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.

Advertisement | page continues below

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.

When can you officially tell the sex of your baby?

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.

Advertisement | page continues below

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.

How to support your baby's genitalia and urinary system development

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.

Key milestones in sex organ and urinary tract development

Weeks pregnantMilestone
4-5 weeksPrimitive internal sex organs start forming.
6-7 weeksTestes begin to form in boys.
8 weeksExternal genitals of boys and girls look the same.
10 weeksPenis begins to form in boys. Kidneys hook up to the bladder.
12 weeksUterus and ovaries are evident in girls. Kidneys start making urine.
20 weeksMale and female sex differentiation is complete. Typical timing for anatomy ultrasound exam, when your baby's genitals can be seen.

Learn more:

Advertisement | page continues below
Follow your baby's amazing development
Sources

BabyCenter's editorial team is committed to providing the most helpful and trustworthy pregnancy and parenting information in the world. When creating and updating content, we rely on credible sources: respected health organizations, professional groups of doctors and other experts, and published studies in peer-reviewed journals. We believe you should always know the source of the information you're seeing. Learn more about our editorial and medical review policies.

Mayo Clinic. Fetal Development: The 1st Trimester. 2022. https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/prenatal-care/art-20045302Opens a new window [Accessed June 2022]

Embryology, Sexual Development. 2022. https://www.ncbi.nlm.nih.gov/books/NBK557601/?report=reader#!po=73.0769Opens a new window [Accessed June 2022]

Institute of Medicine (US) Committee on Understanding the Biology of Sex and Gender Differences. Does Sex Matter? Chapter 3, Sex Begins in the Womb. 2001.https://www.ncbi.nlm.nih.gov/books/NBK222286/Opens a new window [Accessed June 2022]

Louisiana Department of Health. Stage of Fetal Development: First Trimester. Undated. https://ldh.la.gov/page/986Opens a new window [Accessed June 2022]

NHS. You and your baby at 13 weeks pregnant. 2021. https://www.nhs.uk/pregnancy/week-by-week/13-to-27/13-weeks/Opens a new window [Accessed June 2022]

Nova Online. Fertility Throughout Life. 2001. https://www.pbs.org/wgbh/nova/baby/fert_text.htmlOpens a new window [Accessed June 2022]

NHS. 20-week screening scan. 2021 https://www.nhs.uk/pregnancy/your-pregnancy-care/20-week-scan/Opens a new window [Accessed June 2022]

Seminars in fetal and neonatal medicine. Renal development in the fetus and premature infant. 2017. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5387761/Opens a new window [Accessed June 2022]

NIH. Non-Invasive Prenatal Testing (NIPT) for Fetal Sex Determination. Health Technology Assessment. 2016. https://pubmed.ncbi.nlm.nih.gov/29553638/Opens a new window [Accessed June 2022]

Mount Sinai. Amniocentesis. 2021. https://www.mountsinai.org/health-library/tests/amniocentesisOpens a new window [Accessed June 2022]

Cleveland Clinic. Chorionic Villus Sampling for Prenatal Diagnosis. 2021 https://my.clevelandclinic.org/health/diagnostics/4028-chorionic-villus-sampling-for-prenatal-diagnosisOpens a new window [Accessed June 2022]

Seminars in Fetal and Neonatal Medicine. Renal development in the fetus and premature infant. 2017. https://www.sfnmjournal.com/article/S1744-165X(17)30001-X/fulltext [Accessed June 2022]

MedlinePlus. Fetal Development. 2021. https://medlineplus.gov/ency/article/002398.htmOpens a new window [Accessed June 2022]

University of Michigan Medical School. Anatomy, Urinary System. 2000. https://www.med.umich.edu/lrc/coursepages/m1/embryology/embryo/11urinarysystem.htm#:~:text=Urine%20formation%20begins%20towards%20the,happens%20by%20the%20ninth%20week [Updated June 2022]

NIH. Fetal bladder development. 2007. https://pubmed.ncbi.nlm.nih.gov/17999044/Opens a new window [Accessed June 2022]

March of Dimes. Amniotic Fluid. 2020. https://www.marchofdimes.org/pregnancy/amniotic-fluid.aspxOpens a new window [Accessed June 2022]

ACOG. Nutrition During Pregnancy. 2022. https://www.acog.org/womens-health/faqs/nutrition-during-pregnancyOpens a new window [Accessed June 2022]

Maggie Getz

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.

Advertisement
Advertisement