Most parents can find out their baby’s sex between 18 and 22 weeks of pregnancy, during the standard anatomy ultrasound. But depending on the method, you may get an answer much earlier. Blood tests can screen for fetal sex as early as 9 weeks, and ultrasound can sometimes give a reliable reading around 14 weeks once the external genitalia have fully formed.
How Fetal Sex Develops
Genitals start forming around week 7 of pregnancy, but at that stage the structures are identical regardless of sex. By week 10, external genitalia begin to take shape, though they’re still too small and undifferentiated to see on an ultrasound. It isn’t until around week 14 that the external genitalia are fully developed and distinct enough to potentially identify on imaging.
This biological timeline is the reason no ultrasound method is reliable before the late first trimester. The anatomy simply hasn’t caught up yet.
Blood Tests: The Earliest Option
Noninvasive prenatal screening (often called NIPT or NIPS) is a blood draw that analyzes fragments of fetal DNA circulating in your bloodstream. It can screen for chromosomal conditions like Down syndrome and also reveals fetal sex. It’s available as early as 9 weeks, making it the earliest way to find out.
NIPT is highly accurate for sex determination because it’s reading chromosomes directly (XX or XY) rather than interpreting an image. The test is widely available, though not every insurance plan covers it. Your provider may offer it as part of routine first-trimester screening or recommend it based on age or risk factors. Results typically come back within one to two weeks.
Ultrasound at 12 to 14 Weeks
Some practitioners attempt to identify sex during a first-trimester ultrasound, usually performed between 11 and 14 weeks. At this stage, a small structure called the genital tubercle (sometimes called “the nub”) is visible. Technicians look at the angle of this nub relative to the spine: angled upward more than 30 degrees suggests a boy, while a nub that’s flat or angled downward suggests a girl.
This approach, informally known as “nub theory,” has real limitations. In one study of 172 fetuses, the genital region couldn’t be examined at all in about 9% of cases due to fetal position or maternal body composition. Even when the nub is visible, predictions at 11 or 12 weeks are less reliable than those made at 13 or 14 weeks, when the anatomy is more developed. Consider any sex prediction before 14 weeks a preliminary guess rather than a confirmation.
The Anatomy Scan: 18 to 22 Weeks
The anatomy scan is the gold standard for finding out your baby’s sex. Performed between 18 and 22 weeks (most commonly around 20 weeks), this detailed ultrasound evaluates organ development, growth, and physical structures, including the genitalia. Accuracy at this stage runs between 95% and 99%.
Even at 20 weeks, errors are still possible, though rare. Your baby needs to be in a cooperative position with legs uncrossed for the technician to get a clear view. If the baby is curled up or facing the wrong direction, your provider may ask you to walk around or come back for a second look. Most parents who want to know the sex learn it at this appointment.
What Can Make It Harder to Tell
Several factors affect how clearly an ultrasound can identify sex, especially earlier in pregnancy. Fetal position is the most common obstacle. If the baby’s legs are together or the body is turned away from the probe, even a skilled technician may not be able to see what they need.
Maternal weight plays a role too. Ultrasound waves travel through tissue, and more tissue between the probe and the baby means fuzzier images. Research from UT Southwestern Medical Center found that a BMI below about 24 was the best cutoff for accurate gender prediction at 11 to 13 weeks. Above that threshold, the odds of a correct early prediction drop. This matters less at the 20-week anatomy scan, when the baby is larger and the structures are more distinct, but it can still affect image quality.
Placental position can also block the view. An anterior placenta (one attached to the front wall of the uterus) sits between the ultrasound probe and the baby, which sometimes obscures details in early scans.
Invasive Tests That Also Reveal Sex
Two diagnostic procedures, chorionic villus sampling (CVS) and amniocentesis, analyze fetal chromosomes directly and reveal sex with roughly 99% accuracy. CVS is performed between 10 and 13 weeks, and amniocentesis around 16 weeks. Both are invasive, meaning they carry a small risk of complications, so they’re offered for medical reasons like screening for genetic conditions rather than solely to determine sex.
If you’re already having one of these tests for another reason, sex determination comes as part of the results. Otherwise, NIPT or the anatomy scan are the standard paths.
Quick Comparison by Week
- 9 weeks: NIPT blood test can screen for sex; results in 1 to 2 weeks
- 10 to 13 weeks: CVS can reveal sex if performed for medical reasons (99% accurate)
- 12 to 14 weeks: Early ultrasound may give a preliminary prediction, but accuracy varies
- 16 weeks: Amniocentesis can confirm sex if performed for medical reasons
- 18 to 22 weeks: Anatomy scan ultrasound, 95 to 99% accurate, the most common way parents find out
For most pregnancies, the practical answer is that you’ll have your most reliable ultrasound-based answer around 20 weeks. If you want to know sooner and your provider offers NIPT, that pushes the timeline back to as early as 9 or 10 weeks with a simple blood draw.