The sex of a baby is determined at conception, but you can find out what it is as early as six weeks into pregnancy with a blood test, or with near-perfect accuracy on a standard ultrasound after 13 weeks. The method you use depends on how early you want to know and how much certainty you need. Here’s what actually works, what doesn’t, and when each option becomes available.
When Fetal Sex Becomes Visible
A baby’s external genitalia start developing around week 9 of pregnancy, but male and female structures look identical until roughly week 12. That’s why ultrasound predictions before 12 weeks are unreliable. The biology simply hasn’t caught up yet. Blood-based tests get around this limitation by detecting fetal DNA fragments circulating in the mother’s bloodstream rather than looking at anatomy directly.
At-Home Blood Tests (6 to 8 Weeks)
Consumer DNA tests are now the earliest option available. These work by detecting tiny fragments of fetal DNA in a sample of the mother’s blood. If Y-chromosome DNA is present, the baby is male. If none is found, the baby is female. The Peekaboo test, for example, reports 99.5% accuracy starting at six weeks, validated in a clinical study of 215 pregnant women. The SneakPeek test claims 99.1% accuracy at eight weeks, with 100% sensitivity for detecting male pregnancies in its validation study.
There are a few caveats. Before six weeks, fetal DNA levels in the mother’s blood can be too low to detect reliably, which increases the chance of a false “girl” result. These tests are also not validated for twin or other multi-fetal pregnancies, since detecting Y-chromosome DNA would only tell you that at least one baby is male. Most at-home tests involve either a finger-prick blood collection kit or a visit to a local lab for a blood draw, with results returned in a few days.
First-Trimester Ultrasound (11 to 14 Weeks)
If you have an early ultrasound (often done as part of routine screening between 11 and 14 weeks), your sonographer may be able to estimate fetal sex, but accuracy varies dramatically by week. At 11 weeks, studies show correct identification rates as low as 46 to 70%. By 12 weeks, that jumps to 75 to 98.7%. At 13 weeks, accuracy reaches 97 to 100%.
The technique behind these early predictions is sometimes called “nub theory.” Before the genitalia are fully formed, both sexes have a small protrusion called the genital tubercle. In males, it angles upward at more than 30 degrees from the spine. In females, it stays relatively flat. A 2012 study of over 1,200 patients found 96.6 to 100% accuracy using this method starting at 12 weeks, and a 2014 study found 100% accuracy after 14 weeks. The catch: accuracy depends heavily on the baby’s position and the skill of the person reading the scan. Many practitioners won’t commit to a prediction this early.
Second-Trimester Ultrasound (18 to 22 Weeks)
The anatomy scan, typically scheduled between 18 and 22 weeks, is when most parents learn their baby’s sex. By this point, external genitalia are fully developed and clearly visible. Overall diagnostic accuracy at this stage is 99.8% in studies, with virtually no difference in reliability between male and female predictions. This is the standard, no-extra-cost method most people use, and it’s the one your provider will offer unless you specifically request something earlier.
Occasionally the baby’s position makes a clear view impossible, in which case your provider may suggest a follow-up scan. But for the vast majority of pregnancies, a single anatomy scan gives a definitive answer.
Medical Diagnostic Tests
Some prenatal tests designed to screen for chromosomal conditions also reveal fetal sex as a byproduct. These aren’t typically ordered just for sex determination, but if you’re having one done for medical reasons, the information comes along for free.
- NIPT (noninvasive prenatal testing): A blood test available from about 10 weeks onward. It analyzes fragments of fetal DNA in the mother’s blood, similar to consumer tests but as part of a broader chromosomal screening. Sex determination is highly accurate.
- CVS (chorionic villus sampling): Performed between weeks 10 and 13. It involves taking a small sample of placental tissue and analyzing the chromosomes directly. About 99% accurate for all results, including sex. This is an invasive procedure with a small risk of complications, so it’s reserved for pregnancies with specific medical indications.
- Amniocentesis: Done around 16 weeks. A needle draws a small amount of amniotic fluid for chromosomal analysis. Like CVS, it’s essentially 100% accurate for sex determination but carries procedural risks and isn’t done purely for that purpose.
IVF With Genetic Testing
For pregnancies conceived through IVF, preimplantation genetic testing can identify the sex of each embryo before transfer. Because this involves directly analyzing the chromosomes of the embryo itself, accuracy is nearly 100%. This is the only method that can determine sex before pregnancy even begins. In some countries, selecting an embryo based on sex for non-medical reasons is restricted or prohibited.
Folk Methods That Don’t Work
A number of popular theories claim to predict sex based on pregnancy symptoms or the mother’s body. None hold up to scientific scrutiny.
The most persistent myth is that fetal heart rate predicts sex: above 140 beats per minute means girl, below means boy. Studies comparing heart rate patterns between male and female fetuses have found no significant differences in heart rate at any gestational age. Another common claim is that the shape or height of the mother’s belly reveals fetal sex. Belly shape is actually determined by the mother’s muscle tone, number of previous pregnancies, and the baby’s position at any given moment. It has no connection to sex.
Other folk methods, including the Chinese gender calendar, baking soda tests, and food craving interpretation, have no clinical evidence supporting them. They perform at roughly the same level as a coin flip, which makes sense: with only two outcomes, any random guess has a 50% chance of being right.
Choosing the Right Method for You
Your decision mostly comes down to timing and cost. At-home blood tests offer the earliest results (six to eight weeks) but cost $50 to $150 or more out of pocket and aren’t covered by insurance when done purely for sex prediction. A standard anatomy ultrasound at 18 to 22 weeks costs nothing extra if it’s part of your routine prenatal care and is nearly perfect in accuracy. NIPT, if ordered for chromosomal screening, gives you the answer around 10 to 12 weeks with a simple blood draw.
If you’re looking at accuracy alone, every medical method from 13 weeks onward delivers better than 97% reliability. The differences between them are mostly about when in pregnancy the information becomes available and whether the test serves a dual medical purpose.