Blood-Based Tests of Fetal Sex: Are They Accurate?

Many expectant parents are impatient to learn if they are having a boy or a girl. So they may be attracted to new companies, like SneakPeek, which claim to be able to determine your baby’s sex with 99% accuracy early in the first trimester, with only a simple blood draw.

These new tests rely on what is known as cell free fetal DNA (cffDNA)–small fragments of DNA shed from the placenta into your bloodstream.

Unfortunately, according to a 2011 meta-analysis published in JAMA, some of these companies overstate the accuracy of their tests. The meta-analysis found that none of these cfDNA-based tests are accurate before 7 weeks. (The meta-analysis also examined urine-based tests. None of these were accurate.)

Between 7 and 20 weeks, accuracy for these blood-based tests ranged from 95-98%, depending on the specific techniques employed and the baby’s actual sex. (The tests were more accurate when they use a DNA amplification technique known as real-time quantitative polymerase chain reaction.) Only after 20 weeks were the blood-based tests 97-99% accurate.

How Do Blood-Based Tests for Fetal Sex Work?

During pregnancy, small fragments of your baby’s DNA shed from the placenta into your circulation. These fragments cffDNA, first discovered in pregnant women in 1997, are detectable in your blood starting around 7 weeks of pregnancy, and increase in amount throughout pregnancy.

Within hours of giving birth, your baby’s DNA fragments degrade and disappear from your circulation. This is key: If cffDNA were more durable, it would be useless for prenatal tests, as cfDNA from a current pregnancy would be indistinguishable from cffDNA of a past pregnancy.

(Several new blood-based screens for the three most common trisomies, including Down’s Syndrome, also rely on cffDNA. Of these, Harmony and MaterniT21 are perhaps the best known.)

Because cffDNA contains your baby’s genetic material, it can be analyzed to determine your baby’s sex. The fetal sex tests look for the presence of a Y-chromosome, indicating a boy. Its absence indicates a girl.

There are two big benefits of these cffDNA tests compared to traditional genetic tests involving amniocentesis or chorionic villus sampling (CVS):

  1. Blood-based tests can be performed earlier than CVS and amniocentesis. CVS is usually only performed after 11 or 12 weeks. CVS has been linked to a small increase in the risk of limb deformities when performed before 10 weeks. Amniocentesis is typically only performed after 15 weeks.
  2. Blood-based tests are non-invasive. Therefore, unlike CVS and amniocentesis, they do not raise the risk of miscarriage. (The risk of miscarriage from CVS and amniocentesis is slight but real. The risk is typically stated as roughly 1 in 100 for CVS and 1 in 200 for amniocentesis. However, these risk estimates derive mainly from older studies conducted in the 1970s, before ultrasound guidance was standard. A recent meta-analysis found that the current risk is much lower than the older estimates, closer to 1 in 500 for CVS and 1 in 1000 for amniocentesis.)

The Bottom Line

Blood-based cffDNA tests can give you a peek into your baby’s sex, and after 7 weeks, are fairly accurate. Still, because they are not perfectly accurate, there is a non-negligible chance of being misled.

In my opinion, for expectant parents who are at risk of having a child with a sex-linked genetic disorder, these tests are an excellent option. For everyone else, they seem unnecessary. By 13 weeks, after all, you can learn your baby’s sex via an ultrasound with almost perfect accuracy.

As an aside, my skepticism regarding tests using cffDNA to determine fetal sex does not extend to the cffDNA-based screens for the 3 most common trisomies (trisomy 13, trisomy 18, and Down’s Syndrome). These cffDNA-based trisomy screens are extremely accurate, and I plan to discuss them in detail in an upcoming post.


9 thoughts on “Blood-Based Tests of Fetal Sex: Are They Accurate?”

  1. At 16 weeks I had a ultrasound done and the sonographer said it looked like a girl. She put a arrow pointing to the genital are. Two weeks later I got a phone call from the doctor stating that genetic testing shows its a boy. I have had 7 kids already. The way my body feels is that its a girl. Please leave comments.


    1. Hi…please update if you had a boy or a girl?
      My girlfriend had a babyshower sprinkled in pink and gave birth to a healthy boy. This is my 4th and last so gender is on my mind.


  2. At 10weeks and 1day I had the blood test to check the gender, it came back as a girl, but I think and hoping it’s a boy, this will be my fourth child, I already have 2 girls and 1 boy. Can the blood test be accurate since it was that early


    1. If u dont mind me asking did u what did u end up having a boy or girl, i had my blood work done at 12wks and it came back a girl, i feel its wrong but im barely 14wks so i have a while to wait still to be sure..


  3. I had bought the SneakPeek test and it came back boy, the Dr ran a test and it come back girl, can’t seem to get ultrasound, had 2 anatomy ultrasound and the baby kept putting g oot in way, so which test should I go by


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