What determines a baby’s sex at conception?

The genetics behind fetal sex determination can be somewhat complicated. Fortunately, we’re here to break it down for you. To put it simply, chromosomes determine the sex at conception when the sperm meets the egg.

People typically have 23 pairs of chromosomes. The ones responsible for determining biological sex are called the sex chromosomes. Biological females typically have two X chromosomes, while biological males typically have one X chromosome and one Y chromosome.

So, the moment a sperm fertilizes an egg, two things happen:

• The egg delivers an X chromosome
• The sperm delivers either an X or a Y chromosome

If a baby is given two X chromosomes (one from the egg and one from the sperm), its biological sex is female. If the baby is given an X and a Y chromosome, its biological sex is male.

In other words, biological sex is determined in the very first moments of conception, In other words, biological sex is determined in the very first moments of conception—well before you could even ask questions like how early can you take a pregnancy test after sex. However, at this stage, the physical differences that constitute sex aren’t visible yet. The embryo will develop the structures associated with biological sex over the course of the first and second trimesters.

Fetal development of sex characteristics, week by week

While chromosomes are responsible for determining sex at conception, it takes time for the structures associated with sex to be detectable. Male and female embryos look virtually identical for most of the first trimester, when hormones and genes are just beginning to guide fetal development.

Having a clearer sense of this developmental timeline can help parents understand when it’s possible to reliably learn their baby’s sex.

Conception to week 6

In the first few weeks after conception, a fertilized egg (called a blastocyst) becomes an embryo, a process that completes around 9 weeks of pregnancy.

At this stage, the main characteristics of an embryo are:

• Having either XX or XY chromosomes
• Having a premature, undifferentiated reproductive system
• No recognizable sex organs

Scientists sometimes refer to these premature reproductive systems as bipotential gonads. This means they may grow into either testes (in males) or ovaries (in females), depending on their genetic cues during development.

However, the most important thing to note is that embryos look virtually the same at this phase of development.

Weeks 7 – 12

At this stage of the first trimester, fetal development starts to become more visible. Hormones produced by a baby’s early reproductive system become active, encouraging the development of internal and external sex traits.

Around week 9, an embryo also graduates into a fetus. Sex traits around this time undergo the following changes:

• If a fetus has XY chromosomes, testes may begin to form due to the presence of the SRY gene
• If a fetus has XX chromosomes, the uterus and fallopian tubes may start to form
• The organs involved in reproduction start to develop visible differences

Even though reproductive structures begin to form at this stage, they tend to look very similar for several weeks.

This, in part, is why ultrasounds can’t reliably determine a baby’s sex early in pregnancy. The physical structures associated with sex aren’t firmly developed yet, and that’s what ultrasounds rely on.

So, at this stage, only a genetic test based on chromosomal sex (like the SneakPeek® Test) can accurately predict a baby’s sex.

Weeks 13 – 20 and beyond

Weeks 13 through 20 mark the beginning of the second trimester, when external reproductive structures become more defined. At this stage, it becomes more possible for healthcare providers to identify sex on an ultrasound.

This second-trimester ultrasound, called an anatomy scan, typically occurs between 18 and 22 weeks of pregnancy. The purpose of the anatomy scan is to:

• Check on a fetus’s development
• Screen for potential structural issues

While a baby’s sex can be determined at the anatomy scan, this isn’t the main purpose of the ultrasound. Finding out a baby’s sex during the anatomy scan also depends on the baby’s position in utero: Sonographers need to be able to see the baby’s anatomy to discern their sex.

How soon can you know your baby’s sex with confidence?

As mentioned, the anatomy scan typically happens between 18-22 weeks of pregnancy. So, parents who rely on the anatomy scan to learn their baby’s sex have to wait until the second trimester for answers.

Alternatively, DNA-based testing, like the SneakPeek® Test, can provide reliable results much sooner. You can tell the gender of your baby as early as the first trimester.

The SneakPeek® Early Gender Test: Know baby’s sex from as early as 6 weeks

The SneakPeek® Early Gender Test was designed to help parents discover their baby’s predicted sex as early as possible in their pregnancy. The SneakPeek® Test analyzes a small sample of blood from the pregnant parent, scanning for pieces of fetal DNA in the maternal bloodstream.

Here’s how it works:

1. The pregnant parent collects a small blood sample at home using a virtually painless device included in the kit.
2. They mail the sample to a laboratory where scientists screen for tiny fragments of fetal DNA.
3. If a Y chromosome is found in the fetal DNA, the baby is predicted to be male; if no Y chromosome is found, the baby is presumed to be female.
4. Parents are sent their test results securely via email, often as soon as the same day the lab receives their sample.

Clinical studies evaluating the SneakPeek® Test report over 99% accuracy in results when the test is used as directed. This means you can use the SneakPeek® Test to reliably learn your baby’s sex as soon as 6 weeks into gestation.

Should you test early or wait for your anatomy scan?

More freedom in prenatal testing means parents today have a choice: Find out their baby’s sex sooner, or on the traditional timeline?

Anatomy scan testing

As mentioned, the anatomy scan is a standard part of prenatal care that takes place in the second trimester. While its purpose isn’t to reveal the sex of a baby, it’s often possible to do so during the exam.

If you’re willing to wait until your second trimester, you may be happy learning your baby’s sex during this prenatal care milestone.

DNA-based testing

DNA-based testing is a newer option that can take place as early as the first trimester. Its purpose is specifically to predict the sex of a baby.

If you prefer this method of testing, you can look forward to:

• Testing in the comfort and convenience of home
• Earlier results, giving you more time to plan and prepare
• High reliability, with the SneakPeek® Test offering 99.9% accuracy when used as directed

Another bonus? Satisfying your curiosity. If you’re on the edge of your seat and craving more information about your little one, DNA-based testing with the SneakPeek® Test may be the right choice for your family.

Know early, plan early with the SneakPeek® Test

From a biological perspective, sex is determined from the beginning, at conception, along with your baby’s genetic blueprint. While reproductive organs take longer to develop, knowing whether your baby has either XX or XY chromosomes can help you learn whether they’ll be a boy or a girl.

Discover your baby’s sex sooner than ever with the SneakPeek® Early Gender Test. The SneakPeek® Test gives parents the option to test at home or at a healthcare provider’s office and delivers over 99% clinically proven accurate results as soon as 6 weeks into gestation. That’s 12 weeks ahead of the anatomy scan!

Get to know your baby sooner with the #1 OBGYN-recommended at-home early sex prediction test.* Order yours today.

*SneakPeek recommended six to one among 100 OBGYNs surveyed