By Sarina Gomez
This article is general information, not medical advice. Ask your midwife, OB-GYN or sonographer about your own pregnancy and scans.
The earliest reliable way to find out your baby’s sex is a cell-free DNA (cfDNA) blood test, which can be done from about 10 weeks of pregnancy. Many parents, though, find out at the anatomy ultrasound at about 18 to 22 weeks, if the baby is in a good position. CVS and amniocentesis also show the sex, but they’re done for medical reasons, not to learn boy or girl. Here’s every method on one timeline, with what the evidence says about each.
The short answer: a timeline
Your baby’s sex is set at conception by the sex chromosomes, two X chromosomes for a girl and an X and a Y for a boy (ACOG). What changes over the weeks is how early tests can detect it.
| From about | Method | What it is | How it shows the sex |
|---|---|---|---|
| 10 weeks | Cell-free DNA (cfDNA, often called NIPT) | A blood test from the pregnant parent | Looks for Y-chromosome DNA from the placenta |
| 10–13 weeks | Chorionic villus sampling (CVS) | A diagnostic test on a small sample of placenta | Reads the chromosomes directly |
| 12–14 weeks | First-trimester ultrasound | Some sonographers will guess from the angle of the developing genitals | An early visual call, less certain than later |
| 15–20 weeks | Amniocentesis | A diagnostic test on a small sample of amniotic fluid | Reads the chromosomes directly |
| 18–22 weeks | Anatomy scan | The standard mid-pregnancy ultrasound | The sonographer looks at the genitals |
Timings: ACOG on screening, ACOG on diagnostic tests and ACOG on ultrasound.
Cell-free DNA blood test from 10 weeks
cfDNA testing screens for conditions such as Down syndrome and for problems with the number of sex chromosomes. It can be done starting at 10 weeks, and results take about a week (ACOG). Because it looks at the X and Y chromosomes, it can also tell you the sex if you ask.
Why 10 weeks? The test reads small DNA fragments from the placenta in your blood. MedlinePlus Genetics explains that the share of placental DNA usually needs to be above 4 percent, which typically happens around the tenth week.
How accurate is it for sex? A 2011 meta-analysis in JAMA pooled 57 studies of blood-based sex testing. Overall, it detected a boy correctly 95.4% of the time and a girl correctly 98.6% of the time. Testing at 7 to 12 weeks performed nearly as well, but tests done before 7 weeks were unreliable, and so were tests on urine (Devaney et al., 2011). That’s worth knowing before you buy an at-home kit that promises an answer at six weeks.
cfDNA is a screening test for medical conditions first. Ask your provider whether it’s offered or covered for you, and whether you want the sex included in the results.
CVS and amniocentesis
These two are diagnostic tests: they examine the baby’s chromosomes directly, so they also show the sex.
- CVS takes a small sample of tissue from the placenta, usually between 10 and 13 weeks.
- Amniocentesis takes a small amount of amniotic fluid with a thin needle, usually between 15 and 20 weeks.
Both carry a small chance of pregnancy loss, with CVS slightly higher than amniocentesis, according to ACOG. They’re offered to answer medical questions, and nobody should have one just to find out boy or girl. If you’re having one anyway, you can ask whether the report includes the sex.
Ultrasound: early guesses and the anatomy scan
At 12 to 14 weeks. Some sonographers will offer an early guess from the angle of the genital tubercle, the small structure that becomes the genitals. In a study of 656 pregnancies, a guess was possible in 93% of cases; for those confirmed at birth, male calls were 99–100% accurate and female calls ranged from 91.5% at 12 weeks to 100% by 13 weeks (Efrat et al., 2006). A smaller study at 11 to 14 weeks found 85.7% accuracy (Chelli et al., 2009). A review advised against trying before 12 weeks, when it’s relatively inaccurate (Odeh et al., 2009). This is the idea behind “nub theory”, which we cover in detail in nub theory: how accurate is it at 12 weeks?.
At 18 to 22 weeks. ACOG recommends at least one standard ultrasound, usually at 18 to 22 weeks, and notes that “if the fetus is in a good position, it may be possible to tell the sex” (ACOG). The same review found sonographic sex assignment after 13 weeks accurate in 99% to 100% of cases when the genitals are typical.
Not every scan gives an answer. The NHS warns that a sonographer can’t be 100% certain, that an awkward position can make it impossible to tell, and that some hospitals don’t tell parents the sex at all. Tell the sonographer at the start if you want to know, or if you don’t. Our guide to the 20-week anatomy scan explains what else that scan checks.
What doesn’t work
Heart rate, the Chinese calendar, the ring test and the shape of your bump don’t tell you the sex. A meta-analysis found no significant difference in first-trimester heart rate between boys and girls (see baby heart rate and gender), and a study of 2.8 million births found the Chinese gender chart no better than a coin toss. Our post is any baby gender predictor 100 percent accurate? rates every at-home method.
If you’d rather wait
Waiting until birth is a perfectly good choice. If that’s you:
- Say so early at every scan, and ask the sonographer to warn you before showing the area between the legs.
- Ask for the result in a sealed envelope if you might want it later, for a party or a private moment.
- Check your patient portal settings. Some reports list the sex, so ask your provider to leave it out or open the report together later.
If you’d like a reveal moment while you wait, SeeMyBaby’s gender reveal party opens a gift box on your iPhone with a drumroll and pink or blue balloons. It’s a playful AI guess, just for fun, not a medical result, and it won’t tell you the real answer. If you already know the sex, you tell the app and no guess is made. For real parties, our gender reveal ideas start with how to get the sealed result to the person setting up the reveal.
Sources
- American College of Obstetricians and Gynecologists: Prenatal Genetic Screening Tests, Prenatal Genetic Diagnostic Tests and Ultrasound Exams
- MedlinePlus Genetics: What is noninvasive prenatal testing (NIPT)?
- NHS: Ultrasound scans in pregnancy
- Devaney SA, et al. Noninvasive fetal sex determination using cell-free fetal DNA. JAMA. 2011.
- Efrat Z, et al. Fetal gender assignment by first-trimester ultrasound. Ultrasound Obstet Gynecol. 2006.
- Chelli D, et al. Fetal sex assignment by first trimester ultrasound. Prenat Diagn. 2009.
- Odeh M, et al. Sonographic fetal sex determination. Obstet Gynecol Surv. 2009.
More pregnancy guides are on the SeeMyBaby blog.
Questions parents ask
What is the earliest you can find out the gender?
A cfDNA blood test from about 10 weeks is the earliest reliable option. Blood tests done before 7 weeks were unreliable in a large review.
Can you tell the gender at 12 weeks?
Sometimes. Studies of experienced sonographers at 12 to 14 weeks found high accuracy, but calls before 12 weeks are relatively inaccurate, and many clinics don’t offer an early guess.
How accurate is the gender ultrasound at 20 weeks?
When the baby is in a good position, it’s usually very accurate, but the NHS notes it can’t be 100% certain and position can make it impossible to tell.
Can a gender ultrasound be wrong?
Yes, occasionally, especially early or when the view is poor. If the sex matters for a medical reason, your provider may confirm it with another test.
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