An Atypical Finding on the X Chromosome
Your NIPT result came back unclear on the X chromosome. This guide explains what that means, why it happens, and what happens next — in plain language, one idea at a time.
What this guide is
General education for expectant parents who have received an “atypical finding” involving the X chromosome on a cell-free DNA (NIPT) screening test.
What this guide is not
It is not medical advice, and it is not a diagnosis. Your own report, your own clinician, and your own genetic counselor come first.
Dr. Chukwuma Onyeije, MD · Maternal-Fetal Medicine · Atlanta Perinatal Associates · openmfm.org
Three things to know right now
The other conditions came back low risk
Your report states you are at low risk for the other chromosome conditions this test screened for. That part is genuinely reassuring, and it is worth holding on to.
This is not a diagnosis
The word “atypical” means the lab saw something it could not interpret with confidence. It does not mean your baby has Turner syndrome.
There is a clear path forward
A detailed ultrasound, genetic counseling, and — if you choose it — a diagnostic test can give a real answer. Nothing irreversible has to be decided today.
“Atypical” means unclear — not “abnormal and confirmed.” Most of the time, this kind of finding turns out not to be a true finding in the baby.
NIPT is a screening test, not a diagnosis
NIPT — also called cell-free DNA (cfDNA) screening — is a blood test taken from your arm. It examines tiny fragments of DNA floating in your bloodstream and estimates the chance of certain chromosome conditions.
A screening test estimates a likelihood
Like a smoke detector: it warns you early and it is very good at its job, but it can go off when there is no fire. It tells you whether to look closer — not what is actually there.
A diagnostic test gives an answer
Like a fire inspection: a person goes into the building, looks at the actual structure, and reports what is really present. Amniocentesis is a diagnostic test.
Your report is a screening result. Only a diagnostic test can tell you whether your baby’s own chromosomes are affected. That is why the report mentions the option of further testing rather than giving you an answer.
Most of that DNA comes from the placenta
This is the single most important fact in this entire guide. The DNA that NIPT reads is shed mainly by the placenta — the organ that nourishes your baby — and not directly by the baby.
NIPT reads DNA shed by the placenta into your blood.
Usually this works well
The placenta and the baby grow from the same fertilised egg, so they usually have the same chromosomes — which is why NIPT is such a good screening test.
But not always
Sometimes the placenta’s cells and the baby’s cells differ. When that happens, an unusual reading can come from the placenta alone and never involve your baby at all.
Your result, in plain language
Here is what the wording on a report like yours actually means, line by line.
Why the report says “atypical” instead of yes or no
A screening result has to fit a known pattern before the lab can attach a number to it. Yours did not fit one pattern — so there was no honest number to give.
A clear positive result
The pattern matches a known chromosome difference. The lab can calculate a risk estimate and report it.
A clear negative result
The pattern looks typical. The lab reports low risk for the conditions it screened for.
An atypical result — yours
The pattern looks like a mixture, or resembles something the lab cannot classify. There is no valid number to give, so the lab says so instead of guessing.
“We don’t know yet” is an honest result — not a mistake, and not a hidden bad result. Your report is telling you that this particular question needs a different kind of test to answer.
What is mosaicism?
Mosaicism means having two different populations of cells in the same person or the same placenta.
Mosaicism = a mixture of two cell types in one person, one placenta, or both.
Mosaicism may exist only in the placenta, in some of your baby’s cells, or in you. Which of those it is makes a very large difference to what it means — and that is exactly what the next tests are designed to find out.
What is monosomy X?
Monosomy X — also called Turner syndrome — means having one X chromosome instead of two, in some or all of the body’s cells.
What it is not
It is not an illness you catch, and it is not caused by anything either parent did or did not do.
What it can look like
Effects vary widely. Many girls and women with monosomy X have normal intelligence and live full, healthy lives. Mosaic forms generally have milder effects than a complete 45,X.
Five possible explanations for your result
Only the placenta
The placenta carried an unusual cell line. Your baby’s chromosomes are normal.
The baby has mosaic monosomy X
Some of your baby’s cells are missing an X chromosome; others are not.
It comes from you
Some women naturally have a few cells that lost an X, or a small X change.
An unusual piece of an X
A small piece of an X chromosome is missing, extra, or rearranged.
An early twin
DNA from a twin who stopped developing very early can linger in your blood.
When this kind of result is followed up, it most often turns out to be explanation 1, explanation 3, or normal variation — not a true finding in the baby. Explanations 2, 4, and 5 are possible but less common.
The next four slides take each explanation in turn.
It is only in the placenta
The medical name is confined placental mosaicism (CPM). The placenta developed a second cell line. Your baby did not.
The placenta had an extra cell line. The baby’s own chromosomes are normal.
How common
This is the most common reason these results turn out to be nothing serious.
What it means for your baby
Normal chromosomes. No treatment is needed for this finding.
What it changes
Because the placenta was involved, we usually recommend extra growth scans and blood-pressure checks later in pregnancy.
The baby has mosaic monosomy X
In this explanation, some of your baby’s own cells are missing an X chromosome, while other cells are not.
How much it matters depends on two things
- How many of the baby’s cells carry the change
- Which tissues those cells are in
A small proportion of affected cells generally means milder effects. A larger proportion, or involvement of the heart or kidneys, means more monitoring.
Why we cannot tell from the blood test
The NIPT sample came from the placenta, so it cannot show us what is happening inside your baby’s own cells.
This is the one explanation that can be confirmed — or ruled out — only with a diagnostic test such as amniocentesis, together with a detailed ultrasound.
An atypical result makes this explanation possible. It does not make it likely. Most of the time, follow-up testing does not find it in the baby.
The finding may come from you, not your baby
This surprises most parents, and it is one of the most common explanations of all.
What this means
Some women naturally carry a small number of cells that have lost an X chromosome, or a small change on an X chromosome. This becomes more common with age.
It usually causes no health problems at all. Many women go their whole lives without knowing.
Why it appears on your report
NIPT reads DNA in your blood — and some of that DNA is yours, not the placenta’s. A maternal variation can therefore look like a fetal finding.
If your baby’s chromosomes come back normal, a simple blood test on you (a karyotype or microarray) can confirm whether this is the explanation.
Nothing you did, ate, took, or were exposed to caused this. A maternal chromosome variation is simply part of your biology — and it is worth knowing about for your own health record.
Two less common explanations
An unusual piece of an X chromosome
Sometimes a small piece of an X chromosome is missing, extra, or rearranged — or there is a small extra marker chromosome.
This changes how the test’s software reads the X chromosome. It does not necessarily mean classic monosomy X. Some rearrangements are inherited from a healthy parent and cause no problems at all.
An early twin who stopped developing
If your pregnancy began as a twin and one twin stopped developing very early, DNA from that pregnancy can remain in your bloodstream for a while.
If that twin had a sex-chromosome difference, it can be detected on your NIPT even though the surviving baby is unaffected. This is uncommon, but it is real.
These two explanations are among the reasons a detailed ultrasound and a diagnostic test are offered. Ultrasound looks at your baby’s structure; a diagnostic test looks at your baby’s actual chromosomes.
How often does this turn out to affect the baby?
Short answer: far less often than a positive result for trisomy 21. For sex-chromosome findings, the result is usually not confirmed in the baby.
A high-risk result for trisomy 21
This is why a positive trisomy 21 screen is taken so seriously.
An atypical sex-chromosome result like yours
Because the signal so often comes from the placenta or from the mother.
Published figures for this vary a great deal depending on the exact finding, the laboratory, and the population tested. Rather than give you a number that may not apply to you, we are telling you the direction of the evidence — and asking you to get the number that fits your report.
Ask your genetic counselor: “For my specific finding and this specific test, how often is it confirmed in the baby?”
The bars above are illustrative of the contrast in how these results behave. They are not a measurement of your individual risk.
What this result does not mean
- It does not mean your baby has Turner syndrome.
- It does not mean something is definitely wrong with your baby.
- It does not mean you should end or change your pregnancy based on this blood test.
- It does not mean you did anything wrong.
- It does not mean your baby will have health problems.
- It does not mean you are required to have an amniocentesis — that is your choice to make.
- It does not mean your baby’s sex is unknowable, although this result can make the sex reported less certain.
An open question — and there are good, well-established ways to answer it. Your care team can walk through every one of them with you.
Why we do not simply repeat the NIPT
Your report states directly: “Repeat cell-free DNA testing is not recommended.” Here is the reasoning behind that.
Repeating it wastes time
It would cost you weeks of waiting, and some decisions and options are easier earlier in pregnancy. Getting the right test matters more than getting a second version of the same unclear answer.
The pathway moves forward instead
The next steps are a detailed ultrasound and, if you choose to have one, an amniocentesis with chromosome studies — tests that examine your baby rather than the placenta.
Your care pathway at a glance
Five steps. You can pause at any point, ask questions, and take time to decide.
Genetic counseling
Your own report explained, and your options laid out.
Detailed ultrasound
A close look at your baby’s structure and growth.
Decide about amniocentesis
Your choice. For this question, amniocentesis is preferred over CVS.
Laboratory testing
Karyotype, microarray, and sometimes FISH on the sample.
Sometimes we test you too
If your baby’s chromosomes are normal, a blood test on you can explain the result.
Each step is an offer and a conversation, not an obligation. You can decline any step and still receive excellent care.
Genetic counseling
This is a conversation, not a verdict. It is the first step because it makes every step after it easier to understand.
What it is
Time with a specialist trained in genetics and in explaining test results clearly, without rushing you.
What you get
Your report explained line by line; what each option can and cannot tell you; space to ask anything, including “what would you do?”
What to bring
Your report, your questions (there is a checklist near the end of this guide), and — if you wish — someone you trust.
Being offered genetic counseling does not mean something is definitely wrong. It means your team wants you to have the information you need to make your own decisions.
A detailed ultrasound
Ultrasound cannot diagnose a chromosome condition. What it can do is show us features that make the finding more — or less — concerning.
Fluid at the neck
Extra fluid behind the neck, or a fluid collection around the neck, is one of the more common features looked for.
The heart
The heart’s structure and its major vessels, including the left side of the heart and the aortic arch.
The kidneys
Kidney shape and position, including whether the kidneys are joined together (horseshoe kidney).
Growth and long bones
Overall size and the length of the arm and leg bones, measured against expected ranges.
Overall anatomy
A full anatomy survey, plus the amount of fluid around the baby and the placenta’s appearance.
Fluid collections
Swelling or fluid in the body cavities, which would be an important finding if present.
That is genuinely reassuring, and it makes a placenta-only explanation more likely. It is still not a diagnosis.
That raises concern, but it still cannot confirm a chromosome condition on its own. A diagnostic test is still needed before any decision.
Amniocentesis — and why CVS is usually not the right test here
Amniocentesis samples your baby. CVS samples the placenta.
Amniocentesis — usually preferred here
- Usually done at about 15–16 weeks or later
- A small sample of the fluid around your baby, using a fine needle
- The cells in that fluid come from your baby, not the placenta
- Carries a small risk of miscarriage, which your team will discuss with you in numbers
CVS — usually avoided for this question
- Can be done earlier, at about 11–14 weeks
- Takes a tiny sample of the placenta
- But the placenta is exactly the tissue that produced this unclear result
- A CVS result here can be impossible to interpret — it may not answer your question at all
What the laboratory will look for
Three different tests answer three different questions. Often more than one is used together.
Karyotype
A picture of the chromosomes that counts them and examines their structure. It can see missing or extra chromosomes and rearranged pieces.
Ask for an extended cell count — counting extra cells makes low-level mosaicism much less likely to be missed.
Microarray (CMA)
A molecular scan that finds tiny pieces of chromosome that are missing or extra — far too small for a karyotype to see.
Useful for partial X deletions or duplications, and it can detect mosaicism present at low levels.
FISH
A fast, targeted look at specific chromosomes using fluorescent probes. It can give a preliminary answer within a day or two.
Often used to check quickly for X and Y material while the full karyotype is still being processed.
A karyotype can see structural rearrangements that a microarray cannot describe, and a microarray can see tiny changes that a karyotype cannot see at all. Together they give the most complete picture.
Sometimes we check your chromosomes too
If your baby’s chromosomes come back normal, the most likely explanation is that the signal came from you.
What happens
A simple blood test from your arm — a karyotype or a microarray — can look for a small chromosome variation in you.
This is not an accusation or a search for blame. It is simply how the puzzle gets solved, and it completes the picture for your whole family.
Why it is worth knowing
- It explains your NIPT result
- It usually has no effect on your health at all
- It can matter for planning future pregnancies
- It may be relevant for other family members
If your chromosomes are normal too, that points toward a placenta-only explanation — which is the most reassuring outcome of all.
If the baby is not affected
This is what happens most of the time. The finding was confined to the placenta, and your baby’s own chromosomes are normal.
What it means
- Your baby does not have monosomy X
- No treatment is needed for this finding
- The unclear result has been explained
What changes in your care
Because your placenta carried an unusual cell line, we usually recommend extra monitoring later in pregnancy. This is precautionary, not alarming.
What that extra monitoring usually looks like
Third trimester · growth scans
Serial ultrasound scans to follow your baby’s growth, because an unusual placenta can sometimes mean slower growth.
Third trimester · blood pressure and urine checks
Regular monitoring for rising blood pressure, since abnormal placentation slightly increases the chance of preeclampsia.
Delivery planning
Timing and mode of delivery are based on how you and your baby are doing — not on this blood test.
After birth
No special newborn testing is needed for this finding once your baby’s chromosomes are confirmed normal.
If mosaic monosomy X is confirmed
The range of outcomes is wide, and mosaic forms are generally milder than a complete 45,X. Many girls and women with monosomy X have normal intelligence and live full, healthy lives.
The spectrum of what this can look like
What helps most: early monitoring
- Growth and height monitoring
- Heart evaluation, including echocardiography
- Kidney imaging
- Hearing checks
- Hormone monitoring around the age of puberty
What this means for you
If the diagnosis is confirmed, you will not be managing it alone. Your team will plan the next steps with you, and pediatric specialists will be involved from the start.
Most of the associated features are monitorable or treatable, and knowing early is what makes that possible.
If Y-chromosome material is found
Occasionally the testing finds a small amount of Y-chromosome material along with the X-chromosome finding. When that happens, the pathway changes a little — and it becomes more of a team effort.
What it means
Your care team will involve specialists who manage differences in how reproductive organs develop, so that your baby receives the right monitoring from the very beginning.
They will explain what monitoring is needed, when it starts, and what it involves — at a pace you are comfortable with.
Who may be involved
- Genetics and genetic counseling
- Maternal-fetal medicine
- Pediatric endocrinology
- Pediatric surgery or urology
- A pediatric psychologist or social worker, if you would like support
This is a well-established pathway with a plan — not an emergency. Your team will walk you through what happens next, answer your questions, and involve the right specialists at the right time.
If this situation applies to you, ask your care team for a written summary of the plan and the team members involved, so you have it to refer back to.
You do not have to decide anything irreversible based on this blood test.
You can take your time
- You do not have to decide about amniocentesis today
- You can ask the same question more than once
- You can talk it over with your partner, your family, or your faith or community leader
- You can change your mind as new information arrives
Please do two things
- Keep your appointments. The ultrasound and the counseling conversation are what turn an unclear result into real information
- Tell your team how you are feeling. Anxiety is a normal part of this, and support is available if you want it
Common questions, answered
Can you still tell me if it’s a girl or a boy?
Often yes — NIPT usually reports the baby’s sex. But when the X chromosome looks atypical, the reported sex can be less certain, so ultrasound or amniocentesis may be more reliable. Ask your counselor exactly what your report says about sex, and how confident they are in it.
Will this change how I deliver, or mean a C-section?
Not by itself. Delivery planning depends on how you and your baby are doing as the pregnancy progresses — growth, position, placental position, and your blood pressure — not on this blood test.
Is the test just wrong?
Not wrong — unclear. The test did detect a real signal. The open question is where that signal came from: the placenta, your baby, or you. That is a different question from “is the test accurate.”
Will my insurance cover an amniocentesis?
Often it is covered when there is a screening result like this, but coverage varies by plan. Ask your clinic’s financial counselor before you decide, so cost is not part of the pressure you are under.
Do I have to tell my family?
No. This is your private medical information, and you choose who to tell and when. Your care team will not share it without your permission.
What if I choose not to have an amniocentesis?
That is a legitimate choice, and it is yours to make. In that case your team would rely on ultrasound monitoring during pregnancy and plan your baby’s evaluation after birth. Please tell your team your decision so they can plan with you rather than around you.
Could this affect future pregnancies?
Usually not. The exception is if testing finds a chromosome rearrangement that a parent carries — in that case a genetic counselor will explain what it means for future pregnancies and whether other family members should be tested.
Key takeaways
This result is not a diagnosis
“Atypical” means the lab could not give a clear answer — not that something is confirmed.
The other conditions were low risk
That part of your report is genuinely reassuring.
This DNA came mostly from the placenta
That is the reason the answer is unclear — and the reason it is often nothing.
Most of the time it is not confirmed in the baby
Sex-chromosome findings behave very differently from a trisomy 21 screen.
Do not repeat the NIPT
Your report says so directly, and it would only cost you time.
Ultrasound plus amniocentesis gives a real answer
For this question, amniocentesis is preferred over CVS.
Nothing has to be decided today
Ask questions, take time, and bring someone with you if it helps.
Whatever the answer, you will have a team
Confirmed or not, there is a plan — and you will not be managing it alone.
Questions to ask your MFM specialist
Print this page, or take a photo of it. You are entitled to clear answers to every one of these.
- What exactly does my report say, in plain language?
- Which chromosome is involved, and what pattern did the lab see?
- For my specific finding and this specific test, how often is it confirmed in the baby?
- Can you tell me the baby’s sex from this test — and how certain is that?
- What will the detailed ultrasound look for, and when will it be done?
- What is my personal risk from amniocentesis, in numbers?
- When would the amniocentesis be done, and when would results come back?
- If my baby’s chromosomes are normal, will you test my chromosomes too?
- If this turns out to be only in the placenta, what changes in my care for the rest of the pregnancy?
- Who will explain the results to me, and how soon after they arrive?
- What are my options if I decide not to have amniocentesis?
- Can I have a written summary of my report and this plan to take home?
“If you were in my situation, what would you want to know before making a decision?”
Where to learn more
People to ask
- Your genetic counselor — the best first stop for questions about this specific result
- Your maternal-fetal medicine specialist — for ultrasound findings and diagnostic testing
- Your obstetrician or midwife — for the rest of your routine care
If your report came from Natera Panorama
The report itself provides a laboratory genetic counseling line for questions about your result:
niptgc@natera.com · 650-646-9058
You can also ask your own clinic to contact the laboratory on your behalf.
References
- American College of Obstetricians and Gynecologists. Screening for Fetal Chromosomal Abnormalities. ACOG Practice Bulletin No. 226. Obstet Gynecol. 2020;136(4):e48–e69; with the subsequent ACOG practice advisory update on cell-free DNA screening.
- Society for Maternal-Fetal Medicine. Consult Series #74: Cell-free DNA screening for aneuploidies — updated guidance. Am J Obstet Gynecol. 2023.
- Society for Maternal-Fetal Medicine. Prenatal diagnostic testing for genetic disorders. ACOG Practice Bulletin No. 162. Obstet Gynecol. 2016;127(5):e108–e122.
- Wang Y, Chen E, Pan Y, et al. Maternal mosaicism is a significant contributor to discordant sex chromosomal aneuploidies associated with noninvasive prenatal testing. Clin Chem. 2014;60(1):251–259.
- Grati FR, Malvestiti F, Ferreira JCPB, et al. Fetoplacental mosaicism: potential implications for false-positive and false-negative noninvasive prenatal screening results. Genet Med. 2014;16(8):620–624.
- Grati FR, Bajaj K, Zanatta V, et al. Implications of fetoplacental mosaicism on cell-free DNA testing for sex chromosome aneuploidies. Prenat Diagn. 2017;37(10):1017–1027.
- Gravholt CH, Andersen NH, Conway GS, et al. Clinical practice guidelines for the care of girls and women with Turner syndrome: proceedings from the 2016 Cincinnati International Turner Syndrome Meeting. Eur J Endocrinol. 2017;177(3):G1–G70.
- Gil MM, Accurti V, Santacruz B, Plana MN, Nicolaides KH. Analysis of cell-free DNA in maternal blood in screening for aneuploidies: updated meta-analysis. Ultrasound Obstet Gynecol. 2017;50(3):302–314.
This presentation is general education. It is not medical advice, it is not a diagnosis, and it cannot take the place of counseling about your own pregnancy and your own test results. Decisions about your care should be made together with your own clinicians, who know your full history. If you are worried about your health or your baby’s health, contact your care team.
Privacy: this presentation contains no personal health information. It is based on the standard wording of an atypical sex-chromosome finding on cell-free DNA screening and has been written to apply generally, not to any individual patient.
OpenMFM · Dr. Chukwuma Onyeije, MD · Atlanta Perinatal Associates