The most reliable way to find out your baby’s sex during pregnancy is through a blood test called NIPT, which can be done as early as 7 weeks, or through an ultrasound, which becomes accurate from about 12 weeks onward. Several methods exist, each available at different points in pregnancy and with different levels of accuracy.
When Your Baby’s Sex Becomes Detectable
Up until about 9 weeks of pregnancy, male and female genitalia look identical on the developing embryo. Around week 12, the external differences between male and female anatomy become visible for the first time. This biological timeline is why ultrasound-based sex identification doesn’t work well before 12 weeks, and why blood-based tests that read fetal DNA have become popular for earlier answers.
NIPT: The Earliest Option
Non-invasive prenatal testing, commonly called NIPT, is a simple blood draw from your arm. It works by analyzing tiny fragments of your baby’s DNA that naturally circulate in your bloodstream. Because male and female chromosomes differ (XX versus XY), the test can identify sex by looking for the presence or absence of Y-chromosome DNA. It becomes increasingly accurate from 7 weeks of pregnancy onward, though results before 7 weeks are considered unreliable.
The accuracy is remarkably high. A 2017 systematic review of NIPT in singleton pregnancies found sensitivity of 98.9% and specificity of 99.6% for determining fetal sex. NIPT was originally designed to screen for chromosomal conditions like Down syndrome, and sex determination is essentially a byproduct of that analysis. The American College of Obstetricians and Gynecologists recommends that screening for sex chromosome differences be offered as an opt-in option with appropriate counseling beforehand.
One important caveat: NIPT is not recommended for twins or higher-order multiples when it comes to sex chromosome screening, due to limited data on accuracy in those pregnancies.
When NIPT Results Can Be Wrong
False results are rare but do happen. One recognized cause is the “vanishing twin” phenomenon, where an early pregnancy begins as twins but one embryo stops developing. The cells from the demised twin can continue releasing DNA fragments into your bloodstream for up to 7 to 8 weeks after it stops growing. If that twin had different chromosomes than the surviving baby, the test may pick up conflicting genetic signals and return an incorrect result. This scenario occurs in roughly 0.5% of all pregnancies and can lead to discordant test results in about 0.3% of consecutive tests. A detailed ultrasound before the blood draw helps identify whether a vanishing twin might be present.
Ultrasound: What to Expect at Each Stage
Ultrasound is the most common way people learn their baby’s sex, typically at the anatomy scan around 18 to 20 weeks. But sex identification is possible earlier than many people realize, with accuracy that improves week by week.
A study of nearly 500 first-trimester ultrasounds found that accuracy climbed sharply with gestational age. At 11 weeks, correct identification was possible only 71.9% of the time, and in over 40% of cases at that stage, the sonographer couldn’t make a determination at all. By 12 weeks, accuracy jumped to 92%. At 13 weeks, it reached 98.3%, approaching the accuracy of genetic testing. From 12 weeks onward, the combined success rate for correctly identifying sex (when identification was possible) was 94.8%. Accuracy was slightly higher for male fetuses (92.5%) than female (91.2%), though the difference was not statistically significant.
By the 20-week anatomy scan, fetal anatomy is much more developed, and sex identification through ultrasound is highly reliable. Occasionally, the baby’s position makes visualization difficult, and your provider may not be able to confirm sex at that appointment.
Invasive Tests: CVS and Amniocentesis
Chorionic villus sampling (CVS) and amniocentesis are diagnostic procedures that analyze your baby’s actual chromosomes, making them essentially 100% accurate for sex determination. CVS is typically performed around 12 weeks by taking a small sample of placental tissue, while amniocentesis is done around 16 weeks using a sample of amniotic fluid.
These procedures are not done just to find out sex. They’re used to diagnose chromosomal conditions and genetic disorders, particularly X-linked conditions that only affect one sex. Both carry a small risk of miscarriage, so they’re reserved for pregnancies where diagnostic information is medically needed. If you’re already having one of these procedures for another reason, sex determination comes along with the results.
Folk Methods Don’t Hold Up
You’ve probably heard that a faster fetal heart rate means you’re having a girl. A study of 655 first-trimester pregnancies put this directly to the test. The average heart rate for female fetuses was 167.0 beats per minute, and for males it was 167.3 beats per minute. The difference was not statistically significant. The researchers concluded that fetal heart rate during the first trimester has no predictive value for sex.
Other popular predictions, like carrying high versus low, food cravings, or the appearance of your skin, have no scientific backing either. They’re fun to speculate about, but none perform better than a coin flip.
Choosing the Right Method for You
Your options depend largely on how far along you are and what information you’re looking for beyond sex determination.
- 7 to 10 weeks: NIPT is the only reliable option. It requires a blood draw and results typically come back within one to two weeks. Many providers offer it as part of routine prenatal screening.
- 12 to 13 weeks: A first-trimester ultrasound can identify sex with over 90% accuracy, though not every practice offers early sex determination at this stage.
- 18 to 20 weeks: The anatomy scan ultrasound is when most people find out. It’s part of standard prenatal care and doesn’t require any additional testing.
If knowing early matters to you, ask your provider about NIPT at your first prenatal visit. Many insurance plans cover it, particularly for pregnancies considered higher risk due to age or other factors. Some people choose to pay out of pocket for standalone fetal sex blood tests available through private companies, though these vary in quality and should be distinguished from the clinical NIPT offered through your healthcare provider.