Wondering when your baby’s sex is determined is one of the most common questions expectant parents have. The answer starts much earlier than you might think. Your baby’s sex is set at conception, although the visible development of reproductive organs takes place over the first trimester. Thanks to ultrasound and other tests, parents can usually find out their baby’s sex around the midpoint of pregnancy—if the little one is in the right position.

While knowing the sex of your baby is often exciting and part of planning, it’s important to remember that biology is complex. Not all babies fit neatly into the categories of male or female. Intersex births, where reproductive or sexual anatomy does not align with typical definitions, are a natural variation that occurs in a small percentage of births.

Key Takeaways

• Your baby’s sex is genetically determined at conception.
• Ultrasound can usually reveal sex between 18 and 22 weeks of pregnancy.
• Intersex variations remind us that human biology is beautifully diverse.

Embryo Development

Here’s a closer look at how your baby’s sex is determined and when development takes place in the womb.

When Is the Baby’s Sex Determined?

The sex of your baby is set at conception. When the sperm fertilizes the egg, the chromosomes pair to determine sex. An X chromosome from the sperm results in XX (typically female), while a Y chromosome from the sperm results in XY (typically male).

Do Embryos Start as One Sex?

In the earliest stages, embryos are sexually indeterminate, meaning they follow a shared path without distinct sex characteristics. It is not that embryos “start female” but rather that the genetic blueprint for male differentiation requires specific activation. This begins when the SRY gene, usually carried on the Y chromosome, is present.

When Does an Embryo Develop Male or Female Traits?

Between the 7th and 12th weeks of pregnancy, reproductive organs begin to take shape:

• If the SRY gene is present, gonadal ridges develop into testes, which produce testosterone to form male genitalia.
• If the SRY gene is absent, gonadal ridges develop into ovaries, and without testosterone, female genitalia form.

Determining Sex During Pregnancy

For parents, one of the most exciting milestones is finding out whether they are expecting a boy or a girl. This usually happens during routine prenatal testing.

When Can You Tell the Sex of a Baby?

Most parents find out their baby’s sex during the mid-pregnancy ultrasound, typically performed between 18 and 22 weeks. This safe, non-invasive test provides detailed images of your baby and, when conditions are right, can reveal sex with high accuracy.

Earlier in pregnancy, NIPT (Non-Invasive Prenatal Testing) can sometimes detect sex as early as 9 weeks by analyzing fetal DNA in the mother’s blood. While NIPT and ultrasound are both reliable, accuracy increases as pregnancy progresses.

Intersex Variations

Sex development is not always binary. Intersex is a natural variation where a baby is born with reproductive or sexual anatomy that does not fit typical definitions of male or female.

How Common Are Intersex Births?

Research suggests that about 1 in 2,000 births involve intersex traits, though the exact numbers vary depending on definitions and reporting. While not often discussed, these variations highlight the natural diversity of human biology and remind us that sex is not always as straightforward as male or female.

Knowing when your baby’s sex is determined can add meaning to the pregnancy journey, but remember that your child’s health and well-being matter far more than early labels. Embrace the wonder of development as you prepare for parenthood, from the first ultrasound to your baby’s arrival.

As you get ready for your baby, explore our baby bottles for feeding, our soothing pacifiers, and thoughtfully curated gift sets that make preparation simple and stylish.