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PubMed This is a summary of 24 peer-reviewed journal articles Updated
Medical Genetics

What Is the Turner Syndrome NIPT False Positive Rate?

At a Glance

The false positive rate for Turner syndrome (Monosomy X) on NIPT is very high, ranging from 65% to 80%. Because NIPT analyzes placental DNA, factors like confined placental mosaicism can trigger a false alarm. A high-risk screening result should always be confirmed with a diagnostic test like amniocentesis.

In this answer

4 sections

01

Screening vs. Diagnostic Testing

02

Why Are False Positives So Common?

03

Next Steps: Confirming the Diagnosis

04

The Role of Ultrasound

If your Non-Invasive Prenatal Testing (NIPT) screening came back “high risk” for Turner syndrome (Monosomy X), it does not mean your baby definitely has the condition. In fact, the false positive rate for Turner syndrome on NIPT is very high [1]. Studies show that the Positive Predictive Value (PPV) for Monosomy X is only about 20% to 35% [2][3][4]. This means that if you receive a high-risk result, there is a 65% to 80% chance that the result is a false positive and your baby does not have Turner syndrome [5]. Because of this, NIPT is considered a screening tool, not a diagnostic test, and a high-risk result should always be confirmed with additional testing before any medical decisions are made [6][7].

Screening vs. Diagnostic Testing

To understand why NIPT can be inaccurate, it helps to know how the test works. NIPT—also called cell-free DNA (cfDNA) testing—analyzes small fragments of DNA floating in your bloodstream. During pregnancy, most of this DNA comes from you, but a small percentage comes from the placenta (which usually shares the same genetics as the baby).

Because NIPT looks at placental DNA rather than directly testing the baby’s DNA, it is only a screening test [2][8]. It flags a potential risk but cannot provide a definitive diagnosis [9]. Diagnostic tests, such as amniocentesis, are required to know for sure if the baby has Turner syndrome [7].

Why Are False Positives So Common?

There are two primary biological reasons why an NIPT might incorrectly flag a high risk for Turner syndrome:

  • Confined Placental Mosaicism (CPM): Because NIPT tests DNA from the placenta, it can detect chromosomal differences that exist only in the placenta and not in the baby [10][11]. It is possible for the placenta to have some cells missing an X chromosome (Monosomy X) while the baby’s cells are completely typical (46,XX or 46,XY) [12][13].
  • Maternal Mosaicism: As women age, it is common and generally harmless for some of their own cells to lose an X chromosome. Because NIPT analyzes both maternal and placental DNA, the test might detect this missing X chromosome in the mother’s blood and mistakenly attribute it to the baby [14][15][16].

Next Steps: Confirming the Diagnosis

If you receive a high-risk NIPT result, your healthcare provider will likely refer you to a genetic counselor. Genetic counselors are specifically trained to calculate your individualized PPV based on your age and the specific testing brand used. They will also discuss options for diagnostic testing [4].

There are two main prenatal diagnostic tests:

  • Amniocentesis: Usually performed between 15 and 20 weeks of pregnancy, this test analyzes cells shed by the baby into the amniotic fluid. Amniocentesis is considered the “gold standard” for confirming NIPT results for Turner syndrome [9]. Because it tests the baby’s actual cells rather than placental cells, it bypasses the issue of confined placental mosaicism [17][18]. For this reason, many genetic counselors and professional guidelines explicitly recommend amniocentesis over CVS for confirming suspected sex chromosome variations like Turner syndrome.
  • Chorionic Villus Sampling (CVS): Usually performed between 10 and 13 weeks, this test analyzes tissue from the placenta. While it can be done earlier in the pregnancy, CVS may not always resolve a false positive caused by confined placental mosaicism because it tests the very same placental tissue that the NIPT screened [19][20].

Waiting for an amniocentesis between 15 and 20 weeks can be an agonizing “limbo” period. It is completely normal to feel terrified and anxious during this wait. Working closely with your care team and focusing on the reassuring statistics can help you navigate this difficult time.

The Role of Ultrasound

Your doctor will likely recommend a detailed ultrasound to look for physical signs associated with Turner syndrome [21][22].

  • Early Ultrasounds: Scans around 12 weeks can spot markers like a cystic hygroma (fluid buildup at the back of the baby’s neck) [21].
  • Anatomy Scans and Echocardiograms: Detecting specific structural heart defects common in Turner syndrome, such as a bicuspid aortic valve or coarctation of the aorta, usually requires a detailed anatomy scan or a specialized fetal echocardiogram later in pregnancy, typically around 18 to 22 weeks [22][23].

If an ultrasound is completely normal, the likelihood that the NIPT result is a false positive increases significantly [24]. However, a normal ultrasound does not completely rule out Turner syndrome. Many individuals with Turner syndrome have no structural ultrasound anomalies at all and only present with shorter stature or premature ovarian insufficiency later in life. Conversely, if ultrasound markers are present, the chance that the baby truly has Turner syndrome is higher. Even with ultrasound findings, an amniocentesis or postnatal testing (after the baby is born) is still required for a definitive diagnosis [9].

Common questions in this guide

Why is the false positive rate for Turner syndrome so high on NIPT?
NIPT tests DNA from the placenta, not the baby. Sometimes, the placenta has cells missing an X chromosome while the baby does not, a condition called confined placental mosaicism. Additionally, the test can pick up harmless missing X chromosomes in the mother's own DNA and mistakenly attribute them to the baby.
What is the actual chance my baby has Turner syndrome if my NIPT is high risk?
The Positive Predictive Value for Monosomy X on NIPT is generally between 20% and 35%. This means there is a 65% to 80% chance the high-risk result is a false positive and your baby does not have Turner syndrome.
Should I get an amniocentesis or CVS to confirm a Turner syndrome NIPT result?
Many genetic counselors and doctors recommend amniocentesis over Chorionic Villus Sampling (CVS) to confirm Turner syndrome. Because amniocentesis tests the baby's actual cells in the amniotic fluid, it bypasses the risk of confined placental mosaicism that can cause false positives in both NIPT and CVS.
Can a normal ultrasound prove my NIPT result is a false positive?
A normal ultrasound significantly increases the likelihood that your NIPT result is a false positive, but it cannot completely rule out the condition. While ultrasounds can detect physical signs like a cystic hygroma or heart defects, a diagnostic test is still required for a definitive answer.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.What was the Positive Predictive Value (PPV) specific to my test results and age?
  2. 2.Because of the high chance of a false positive, do you recommend amniocentesis over CVS for diagnostic confirmation?
  3. 3.Does my early ultrasound show any physical markers, like a cystic hygroma, that might increase the likelihood of Turner syndrome?
  4. 4.Should I be referred for a specialized fetal echocardiogram later in pregnancy to check for heart anomalies?
  5. 5.Could my own genetic makeup (maternal mosaicism) be influencing these test results, and do you recommend any testing for me?
  6. 6.Can you refer me to a genetic counselor to help interpret these results and navigate the next steps?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

Related questions

Does Turner Syndrome Affect Intelligence and Learning?Do Girls With Turner Syndrome Need Estrogen for Puberty?Does Turner Syndrome Cause Hearing Loss & Ear Infections?Why Use Estrogen Patches for Turner Syndrome?How Does the Missing SHOX Gene Cause Short Stature?How to Transition to Adult Care with Turner SyndromeIs Turner Syndrome Inherited or Caused by Parents?Why Do Turner Syndrome Patients Need Kidney Ultrasounds?How to Find Turner Syndrome Support Groups & ResourcesCan You Get Pregnant with Turner Syndrome? Options & RisksWhat Autoimmune Diseases Are Linked to Turner Syndrome?Classic vs. Mosaic Turner Syndrome: What is the Difference?What Is the Average Life Expectancy with Turner Syndrome?When to Start Growth Hormone for Turner SyndromeY-Chromosome in Turner Syndrome: What Is the Risk?Why Do Babies With Turner Syndrome Have Swollen Hands?Why Is Lifelong Heart Monitoring Needed in Turner Syndrome?Why is Pregnancy High-Risk with Turner Syndrome?Why is Turner Syndrome Diagnosis Delayed Until Adolescence?

References

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This page provides educational information about NIPT screening for Turner syndrome. Always consult your obstetrician or a genetic counselor to discuss your specific prenatal test results and diagnostic options.

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