Amniocentesis is classified as a “definitive test,” but no test can guarantee a 100% accurate diagnosis. In rare cases, factors related to mosaic-type Down syndrome or the cell-culture process can produce a result that differs from the diagnosis made after birth. Dr. Hiroshi Oka, Director General of Hiro Clinic NIPT, uses amniocentesis for Down syndrome as an example to explain why these results can be hard to interpret, and how amniocentesis differs from NIPT.
💡 What this article covers
- The main reasons amniocentesis can produce a result that feels like a “misdiagnosis”
- The difference between true and pseudo-mosaicism, and why judging it can be difficult
- Other causes of difficult results, such as failed culture or multiple pregnancy
- How NIPT and amniocentesis differ, and what role each test plays
- Where to turn if a result differs from the diagnosis, and cost support for amniocentesis
Types of Down syndrome and patterns of amniocentesis misdiagnosis
Amniocentesis is a definitive test that examines chromosomes directly, but depending on the “type” of chromosomal abnormality, the result can be hard to interpret. Let’s start by looking at how Down syndrome is classified.
Chromosomal abnormalities in Down syndrome are classified as trisomy (standard type): 94%, translocation type: 5%, and mosaic type: 1%. Of these, the mosaic type is the main factor that makes amniocentesis results difficult to interpret.
With mosaicism, the amniocentesis result and the diagnosis at birth rarely differ. Chromosomal mosaicism comes in two forms: true mosaicism and pseudo-mosaicism. When I explain this difference to patients in clinic, I always show them the diagram and tell them, “the meaning is completely different depending on which type it is.”
Pseudo-mosaicism and true mosaicism
Pseudo-mosaicism and true mosaicism have completely different implications for the baby. Let’s start by clarifying the difference between the two.
Pseudo-mosaicism occurs during the culture process after the amniotic fluid sample is collected, and the fetus itself is normal. Abnormal-looking cells simply appear by chance during cell division in culture; the baby’s own chromosomes have no problem.
With true mosaicism, the fetus itself has both chromosomally abnormal cells and normal cells. If both types are found during amniocentesis, a mosaic-type diagnosis can be made at that stage.
However, if only normal cells grow during culture, or if both types grow but only normal cells are detected, the baby’s chromosomal mosaicism may not come to light until after birth. In other words, even when amniocentesis reports a normal result, the baby can still be diagnosed with Down syndrome after delivery. The reverse can also happen: the case is actually pseudo-mosaicism with no problem for the baby, but the judgment takes longer, prolonging an anxious wait.
Multiple pregnancy
With multiple pregnancies, such as twins or triplets, the judgment can also become more complex. This relates to the technical challenge of the collection procedure itself.
In multiple pregnancies, a sample intended for one fetus may, in theory, actually come from the fetus already tested, which could lead to a mistaken diagnosis for the affected baby. Amniotic fluid collection in multiple pregnancies is performed while checking each fetus’s amniotic sac under ultrasound, but the position of the placentas can make collection more difficult in some cases.
When culture doesn’t go smoothly, and maternal cell contamination
Technical limitations of cell culture, and very rare sample contamination, are other causes of a difficult judgment. These are uncommon, but worth knowing about.
If fetal cells in the amniotic fluid don’t multiply enough, the rapid method (a simplified test that gives results within a few days) cannot produce a judgment. In that case, the standard culture method is used instead, and you wait for a definitive diagnosis. Culture can take longer depending on individual differences in cell growth speed and how many living cells were present in the collected sample. When culture doesn’t go well, the usual next step is a repeat puncture (collecting amniotic fluid again) to check once more. This isn’t common, but it is a real possibility. There is also a small, non-zero chance that a tiny amount of maternal cells get mixed in during collection and affect the judgment. If your doctor tells you “the culture is taking longer than expected,” it helps to know this background.
NIPT vs. amniocentesis: why is amniocentesis called a “definitive test”?
NIPT and amniocentesis occupy fundamentally different positions as tests. Confusing the two makes it easy to misunderstand how their accuracy should be interpreted, so let’s clarify this here.
Reports from Japan’s Ministry of Health, Labour and Welfare expert committee, and academic society guidelines on NIPT, broadly classify prenatal tests as either “definitive tests” or “non-definitive (screening) tests.” NIPT, which estimates risk from the mother’s blood, is a non-definitive test. Amniocentesis, which directly examines cells in amniotic fluid collected by inserting a needle into the abdomen, is positioned as a definitive test. The non-definitive testing offered by Hiro Clinic NIPT has been reported to achieve high detection accuracy for target conditions such as trisomy 21, 18, and 13. However, because it remains a non-definitive test, a positive result still needs to be confirmed by amniocentesis.
| Item | NIPT | Amniocentesis |
|---|---|---|
| Type of test | Non-definitive test (screening) | Definitive test (diagnostic confirmation) |
| Method | Maternal blood draw | Needle inserted into the abdomen to collect amniotic fluid |
| Timing | From around 10 weeks of pregnancy (varies by facility) | Generally from 15-16 weeks of pregnancy |
| Time to results | As fast as 2-5 days (at our clinic) | About 2-3 weeks |
| Physical burden / risk | Blood draw only, no miscarriage risk | Miscarriage risk of about 0.3% (roughly 1 in 300) |
Because amniocentesis examines chromosomes directly, its accuracy is high, but given the mosaicism and culture limitations described above, it isn’t accurate to say that “a definitive test = 100% free of error.” The question of whether a negative amniocentesis result can still miss something comes from this same underlying mechanism.
Misconceptions about amniocentesis accuracy: the risk of assuming it’s “always right”
Amniocentesis is a very accurate test, but placing too much faith in the word “absolute” can leave people more shaken if an unexpected result occurs. Real accounts from people who have had the test reflect this.
On X (formerly Twitter), one user shared how it felt to wait for a definitive diagnosis after the rapid method came back inconclusive. User @krshsikknsti1 posted (in Japanese): “The rapid amniocentesis method came back inconclusive, so I’ll get the definitive diagnosis after culture next week. I’ve been told all outcomes — negative, positive, or mosaic Down syndrome — are still possible.” The fact that the rapid method alone doesn’t always give a conclusive answer is something I explain to patients repeatedly in clinic. Please think of the rapid method’s few-day result and the definitive diagnosis, which takes several weeks, as two separate things.
Another user described experiencing the limits of amniocentesis itself. @kiyuchang wrote (in Japanese): “There was nuchal translucency and quite a bit of swelling, so I went straight to amniocentesis, which was negative. Later they found a cleft lip/palate and a heart condition; the baby was born by normal delivery, and it turned out to be a gene mutation rather than a chromosomal issue — so prenatal diagnosis really isn’t everything.” Amniocentesis examines chromosomal abnormalities. It does not cover gene-level conditions or conditions caused by something other than the chromosomes. Knowing this distinction can change how you interpret a result.
If the result and the diagnosis seem to differ: what you can do after birth
If the amniocentesis result and your baby’s condition after birth seem to differ, the next step is to speak with your doctor or the testing institution. You don’t need to work through this alone.
If chromosomal mosaicism is suspected after birth, it can sometimes be confirmed with additional blood or tissue testing. Genetic counseling with a board-certified clinical geneticist helps sort out what the tests can and cannot tell you, and then works with you to think through childcare and medical support going forward. At Hiro Clinic NIPT, obstetricians, pediatricians, and clinical geneticists work together to provide consultation support both before and after testing.
Explaining your situation to a specialist makes things easier to sort through than carrying the worry alone. Working through what to check next, one item at a time, ends up being a real source of support. Don’t carry it alone — start with a consultation. If questions remain about a postnatal test result, contact the hospital where you delivered, or the genetic counseling desk at the facility where the test was performed.
Deciding whether to have amniocentesis: cost and support programs
Amniocentesis is not covered by insurance and is paid out of pocket, with cost varying by medical institution. Knowing about programs that ease the financial burden can make the decision easier.
Hiro Clinic NIPT offers a cost support program for patients undergoing amniocentesis, with three plans. The Light plan covers up to 100,000 yen for a 3,300 yen fee; the Standard plan covers up to 200,000 yen for a 5,500 yen fee. The Wide plan covers up to 300,000 yen for an 11,000 yen fee, and can also be applied to amniocentesis performed at another clinic. If you’d like more detail, see our articles on when and how amniocentesis is performed and how it differs from microarray chromosome testing.
Frequently Asked Questions (FAQ)
Here are answers to questions we’re often asked in clinic about the accuracy of amniocentesis and the possibility of a misdiagnosis.
Can a baby be diagnosed with Down syndrome after birth even though amniocentesis was “negative”?
In very rare cases, yes. With mosaic-type Down syndrome, only normal cells may be detected during culture, and cases have been reported where the diagnosis only becomes clear after birth.
What is the difference between true mosaicism and pseudo-mosaicism?
Pseudo-mosaicism arises during the culture process, and the fetus itself is normal. True mosaicism refers to a state where the fetus itself has both abnormal and normal cells.
Which is more accurate, amniocentesis or NIPT?
They serve different purposes. Amniocentesis is a definitive test that examines chromosomes directly; NIPT is a non-definitive test that estimates risk from maternal blood. If you want a confirmed diagnosis, amniocentesis is required.
What happens if amniocentesis results are difficult to judge?
If the rapid method cannot produce a judgment, the next step is to wait for a definitive diagnosis using the standard culture method. If culture doesn’t go well, a repeat puncture is used to check again.
Does amniocentesis carry a risk of miscarriage?
Because it involves inserting a needle into the abdomen, the reported risk of miscarriage and similar complications is about 0.3%, or roughly 1 in every 300 procedures.
Is there financial support for the cost of amniocentesis?
Yes. There is a support program covering up to 100,000 yen for a 3,300 yen Light plan fee, up to 200,000 yen for a 5,500 yen Standard plan fee, and up to 300,000 yen for an 11,000 yen Wide plan fee, which can also be applied to amniocentesis performed at another clinic.
Summary
Amniocentesis is highly accurate as a definitive test, but factors such as mosaicism, the limits of cell culture, and sample management in multiple pregnancies can, in very rare cases, produce a result that differs from the eventual diagnosis.
There’s no need to feel anxious simply because of the word “misdiagnosis.” Amniocentesis is not “absolute” — understanding how it works helps you make an informed judgment. What matters is thinking through the next step together with your doctor or a specialist. If you’d like to understand how NIPT and amniocentesis are each positioned, see Everything about NIPT and Down syndrome: accuracy, probability, and the realities of care; if you’d like the basics of amniocentesis, see The basics of NIPT and amniocentesis. If you’re unsure how to read the numbers or how to interpret a result, please feel free to reach out.
Supervised by: Dr. Hiroshi Oka — Director General and Lab Director, Hiro Clinic, Fukumikai Medical Corporation. Graduate of Keio University School of Medicine. Passed the national medical licensing examinations in both Japan and the United States, and holds a medical doctorate. He is one of a small number of physicians in Japan to hold Lab Director certification. Author of “The First Book to Read When You Learn You’re Pregnant.” This article has been prepared with reference to information from public institutions and academic societies, in accordance with medical advertising guidelines. Amniocentesis is a definitive test, but in some cases, such as mosaicism, judgment can be difficult. NIPT is a non-definitive test, and a definitive diagnosis requires amniocentesis. Figures cited may vary by literature and institution; please consult your physician for final decisions on diagnosis and treatment.
References: Ministry of Health, Labour and Welfare, “Expert Committee Report on Prenatal Testing including NIPT” (Japanese) / Japan Society of Obstetrics and Gynecology, “Guidelines on NIPT” (Japanese)
/ Prenatal Testing Accreditation System Steering Committee, “What is NIPT” (Japanese) / Keio University Hospital KOMPAS, “Amniocentesis” (Japanese)
日本皮膚科学会 皮膚科専門医／日本医師会 産業医／東京衛生検査所 指導監督医
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