At What Week Can You Tell the Sex of a Baby?

A blood test analyzing fetal DNA in the mother’s bloodstream can reliably identify a baby’s sex from about seven weeks of pregnancy onward, though most providers run it between ten and twelve weeks. Ultrasound catches up a few weeks later, reaching near-perfect accuracy by thirteen weeks. The gap between those two methods, and the surprising number of things that can complicate the answer, makes the timeline worth understanding in more detail.

What Is Happening Biologically in the First Trimester

The chromosomal sex of a baby is fixed at conception, but the physical structures that ultrasound relies on take time to develop. At eight to nine weeks of gestation, male and female fetuses look identical externally. Both have a small nub of tissue called the genital tubercle, and at this stage it has not yet started growing into recognizably different anatomy. Visible differences between male and female external genitalia begin appearing at around ten weeks, and full development of the penis or clitoris is not complete until roughly seventeen to eighteen weeks.¹

This explains why ultrasound-based sex determination improves dramatically week by week during the first trimester. The anatomy is literally in the process of forming, and a scan at eleven weeks is trying to read a structure that is only partway through its transformation. Blood-based testing sidesteps this problem entirely because it reads chromosomes rather than anatomy.

Blood Tests That Can Give an Answer Before Ultrasound

Non-invasive prenatal testing, commonly called NIPT, works by picking up fragments of fetal DNA circulating in the mother’s blood. These cell-free DNA fragments come from the placenta and carry the baby’s genetic information, including whether a Y chromosome is present. The diagnostic accuracy of this approach for fetal sex is very high, and maternal blood samples taken at seven weeks or later provide more reliable results than those drawn earlier.² One review found the sensitivity and specificity of cell-free DNA testing approached 100% from eight weeks of gestation onward.³

In practice, most NIPT is performed between ten and twelve weeks because the primary reason people get the test is screening for chromosomal conditions like trisomy 21. Sex determination is often reported as a secondary result. In one Australian survey, about four in five NIPT users learned fetal sex through the test, and roughly one in five said sex determination was either the main reason or equally important to other reasons for getting NIPT.³ So while the technology works earlier, the timing in the real world is shaped by the test’s broader purpose.

Ultrasound Accuracy Week by Week

If you are relying on ultrasound alone, the accuracy curve during the first trimester is steep. One well-known study measured accuracy at 11 weeks at just over 70%, climbing to about 99% at 12 weeks and hitting 100% at 13 weeks.⁴ Another first-trimester study found that sex assignment was possible in about 90% of fetuses and accurate in roughly 86% overall, with accuracy improving as gestational age increased.⁵

A separate large study reported that sonographic sex determination from twelve weeks onward had a correct-diagnosis rate of 99.9% after postnatal confirmation across nearly 1,900 pregnancies.⁶ By the second trimester, the picture is even clearer. One audit of routine obstetric ultrasound found that sex assignment was possible in over 99% of second-trimester scans, with correct identification in every case where a determination was made.⁷

First-trimester sex determination on ultrasound relies on the angle of the genital tubercle rather than direct visualization of a penis or labia. One recent study found that a tubercle angle above about 28 degrees reliably indicated a male fetus, with sensitivity of 92% and specificity of 85% using that cutoff.⁸ This method requires a skilled sonographer and a cooperative fetal position, which is why results vary more at this stage.

Why Boys Are Harder to Call at Eleven Weeks

There is an interesting asymmetry in first-trimester errors. At eleven weeks, male fetuses were wrongly assigned as female in 56% of cases, while only 5% of females were incorrectly called male. By twelve weeks, the male error rate dropped to 3%, and by thirteen weeks it was zero.⁴ The reason is anatomical timing. In males, the genital tubercle needs to rotate upward significantly to form the penis, and at eleven weeks that rotation is often not yet pronounced enough to distinguish from the female angle. In females, the tubercle stays relatively flat from an earlier stage, making it easier to identify correctly sooner.

This means if you get a very early ultrasound and are told “girl,” the odds of that being correct are better than if you are told “boy” at the same stage. By twelve to thirteen weeks, the gap disappears and both sexes are identified with comparable reliability.

When Body Size or Fetal Position Gets in the Way

Maternal body mass index can affect both types of testing, though in different ways. For ultrasound, higher BMI makes it harder to get a clear image. One study identified a BMI below about 24 as a cutoff for optimal sex prediction accuracy in the first trimester, with accuracy declining above that threshold.⁹ Fetal position matters too. A baby lying in a way that obscures the lower pelvis can make sex determination impossible at any gestational age, though this is more of a nuisance than a real barrier since it usually resolves at a follow-up scan.

For cell-based approaches to prenatal testing, higher maternal BMI is associated with fewer fetal cells recovered from maternal blood. Women in the normal-weight range had roughly twice as many recoverable trophoblast cells per milliliter of blood as women classified as obese, and no cells at all were recovered from about 16% of samples from women with obesity compared to about 6% in the normal-weight group.¹⁰ Standard cell-free DNA tests (which measure DNA fragments rather than intact cells) are less affected by BMI, but providers sometimes note that the fetal fraction of DNA in maternal blood can be lower in women with higher BMI, which occasionally leads to inconclusive results that require retesting.

The Fetal Heart Rate Myth

One of the most persistent pieces of pregnancy folklore is that you can predict the baby’s sex by fetal heart rate. The usual claim is that a heart rate above 140 beats per minute means girl, and below 140 means boy. This has been studied repeatedly and does not hold up. A study of over 650 first-trimester pregnancies found no meaningful difference: the average heart rate was about 167 beats per minute for both sexes.¹¹

A large retrospective study using NIPT-confirmed sex found a mean heart rate of 165 beats per minute for females and 166 for males, concluding that the difference was clinically negligible and could not predict sex.¹² A systematic review and meta-analysis pooling results from multiple studies reached the same conclusion: even though male fetuses showed very slightly faster heart rates in some data, the difference was too small to have any predictive value.¹³ If your provider tells you the heart rate and someone at a party tries to interpret it for you, they are working from a myth.

When Blood Test and Ultrasound Give Different Answers

With NIPT now routinely reporting chromosomal sex and ultrasound providing anatomical sex at a later scan, discordant results do sometimes surface. The estimated prevalence is roughly 1 in 1,500 to 2,000 pregnancies.¹⁴ When the blood test says one sex and the ultrasound shows another, it triggers a workup that can lead to important diagnoses.

One cohort of nearly 13,000 women identified seven cases where the genetic sex on NIPT differed from the anatomical sex seen on ultrasound or at birth. Several of those cases turned out to involve differences of sex development, conditions where the baby’s chromosomes and anatomy do not align in the usual way.¹⁵ In a separate review, six out of ten prenatally detected cases of differences of sex development were initially flagged because cell-free DNA results did not match what the ultrasound showed.¹⁶

These situations are uncommon, but they illustrate that sex determination is not always a binary question with a clean answer. Atypical genitalia seen on ultrasound can also be a false alarm. In one study, atypical genitalia were confirmed after birth in 64 to 91% of suspected cases depending on the cohort, meaning a significant minority of prenatal flags turned out to be normal anatomy that was hard to interpret on imaging.¹⁷

Vanishing Twins and Other Sources of Confusion

A vanishing twin pregnancy, where one embryo stops developing early on while the other continues normally, can confuse cell-free DNA results. The placental tissue from the nonviable twin may still shed DNA into the mother’s blood, and if that twin had a different chromosomal sex, the test can pick up Y-chromosome DNA from a twin that no longer exists. One study found that vanishing twin pregnancies had sex discrepancy rates of about 10%, compared to just 0.05% in straightforward singleton pregnancies.¹⁸ Individual case reports have documented the same phenomenon, with false-positive Y-chromosome detection attributed to a vanishing twin.¹⁹

Other potential confounders include a recent blood transfusion (donor blood carrying male DNA), a recent pregnancy loss, or in rare cases a maternal condition affecting sex chromosomes. These are uncommon enough that most people will never encounter them, but they are worth knowing about if you get an unexpected result. In cases involving a known vanishing twin, some researchers have found that waiting and retesting allows the contaminating DNA to fade, eventually producing a correct result.¹⁸

Direct-to-Consumer Gender Tests

Some companies have marketed at-home tests that claim to identify fetal sex from a blood sample as early as five weeks of pregnancy. These products exist in a regulatory gray area. Unlike NIPT ordered through a healthcare provider, direct-to-consumer genetic tests for fetal sex have faced criticism for a lack of independent validation of their accuracy claims. The availability of such tests has been flagged as highlighting gaps in the oversight framework for genetic testing.²⁰ At five weeks, fetal DNA levels in maternal blood are extremely low, and even validated clinical NIPT performs better after seven weeks. A healthy dose of skepticism toward any product promising reliable results that early is warranted.

The Ethics of Knowing Early

The ability to determine fetal sex earlier and more easily has raised ethical questions in many countries. The central concern is that early, accurate sex determination via NIPT could facilitate sex-selective termination of pregnancy. This practice, driven overwhelmingly by son preference, has had measurable demographic effects in countries like India and China.²⁰

Professional bodies have weighed in with varied positions. Some ethicists argue that all non-medical sex selection, including choosing for “family balancing,” carries sexist implications because it ties a child’s value to their sex. Others counter that wanting children of both sexes is not inherently discriminatory and does not prevent parents from valuing each child as an individual.²¹ Some commentators have recommended that cell-free DNA testing should not be used for sex determination unless there is a medical reason, such as screening for X-linked genetic conditions, and that testing should ideally be designed to avoid generating sex information when it is not clinically needed.²²

In practice, policies differ widely. Some countries restrict disclosure of fetal sex before a certain gestational age or prohibit it unless medically indicated. Others leave it entirely to the discretion of the provider and patient. In the Australian survey mentioned earlier, the overwhelming majority of respondents, over 92%, said they did not consider sex-selective termination morally acceptable, even as most of them had voluntarily learned their baby’s sex through NIPT.³ The tension between personal curiosity and population-level consequences is unlikely to resolve neatly, especially as testing becomes cheaper and more accessible.

When Sex Determination Has Medical Stakes

For most families, learning the baby’s sex is a matter of personal curiosity or planning. But there are pregnancies where knowing early is medically meaningful. X-linked genetic conditions, like hemophilia or Duchenne muscular dystrophy, primarily affect males. If a family carries one of these conditions, early sex determination through NIPT allows providers to plan targeted diagnostic testing and counseling rather than waiting until anatomy can be assessed on ultrasound weeks later.

Congenital adrenal hyperplasia is another condition where early sex identification matters. In affected female fetuses, excess androgens can cause atypical genital development. Knowing the chromosomal sex early can inform decisions about prenatal treatment. And as prenatal detection of differences of sex development becomes more frequent through the combined use of NIPT and high-resolution ultrasound, expert centers are seeing more cases prenatally than they used to, though consensus on how to manage these cases during pregnancy is still evolving.²³ The technology that satisfies curiosity for most families turns out to be a diagnostic tool of real consequence for a small but important subset.

1
PubMed Central. Macroscopic Whole-Mounts of the Developing Human Fetal Urogenital-Genital Tract: Indifferent Stage to Male and Female Differentiation
2
PubMed. Non‐Invasive Prenatal Testing (NIPT) for Fetal Sex Determination
3
PubMed Central. Regulating non-invasive prenatal testing (NIPT) for fetal sex determination – Section: III. ACCESS TO NIPT INTERNATIONALLY AND WITHIN AUSTRALIA
4
PubMed. First-trimester determination of fetal gender by ultrasound
5
PubMed. Fetal sex assignment by first trimester ultrasound: a Tunisian experience
6
American Journal of Obstetrics and Gynecology. Sonographic determination of fetal sex from twelve weeks of gestation
7
PubMed Central. Accuracy of sonographic fetal gender determination: predictions made by sonographers during routine obstetric ultrasound scans
8
PubMed Central. The Predictive Accuracy of Anogenital Distance and Genital Tubercle Angle for First-Trimester Fetal Sex Determination
9
PubMed. Accuracy of fetal sex determination on ultrasound examination in the first trimester of pregnancy
10
PubMed Central. The effect of maternal body mass index and gestational age on circulating trophoblast yield in cell-based noninvasive prenatal testing
11
PubMed Central. First trimester fetal heart rate as a predictor of newborn sex
12
Sonography. Testing the Fetal Heartbeat Myth: A Large Cohort Study Correlation With NIPT
13
PubMed Central. The role of fetal heart rate in first trimester sonograms in prediction of fetal sex: a systematic review and meta-analysis
14
PubMed. Discordant fetal sex on NIPT and ultrasound
15
PubMed. Sex discordance identification following non-invasive prenatal testing
16
PubMed Central. Prenatal detection and evaluation of differences of sex development
17
PubMed. Prenatal ultrasound finding of atypical genitalia: Counseling, genetic testing and outcomes
18
PubMed. Should vanishing twin pregnancies be systematically excluded from cell-free fetal DNA testing?
19
PubMed. Vanishing twin as a potential source of bias in non-invasive fetal sex determination: a case report
20
PubMed Central. Sex selection and non-invasive prenatal testing: A review of current practices, evidence, and ethical issues
21
Human Reproduction. ESHRE Task Force on ethics and Law 20: sex selection for non-medical reasons
22
Academic Press. Ethics of Cell-Free DNA-Based Prenatal Testing for Sex Chromosome Aneuploidies and Sex Determination
23
PubMed. Prenatal care for Disorders/Differences of Sex Development: 10-years look back in an expert-center