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PubMed This is a summary of 22 peer-reviewed journal articles Updated
Clinical Genetics · Trisomy 18

How Do I Understand My Baby's Trisomy 18 Genetic Report?

At a Glance

A trisomy 18 genetic report explains which chromosome changes were found and whether the pattern is full, mosaic, partial, or translocation. Any mosaicism percentage applies only to the tested sample, so the medical team must combine the result with examinations and other tests to understand the baby's needs.

In this answer

6 sections

01

Screening vs. Diagnostic Tests

02

How the Testing Was Done

03

Decoding the Chromosome Notation (ISCN)

04

Understanding Cell Counts and Mosaicism

05

How the Extra Chromosome is Described

06

What This Report Can and Cannot Tell You

A genetic report describes exactly what was found in the tested cells. Receiving this document can be overwhelming and frightening, especially since the laboratory language is highly technical. To make sense of the results, it helps to identify whether the test was a screen or a diagnosis, what tissue was used, and how the chromosomes are described.

Screening vs. Diagnostic Tests

The first step is determining the goal of the test:

  • Screening Tests (cfDNA/NIPT): Non-invasive prenatal testing analyzes cell-free DNA from the placenta floating in the pregnant parent’s blood [1]. A positive screen suggests a high chance of trisomy 18, but it is not a final diagnosis [2]. It requires confirmation.
  • Diagnostic Tests: These analyze cells directly from the baby or pregnancy. They include prenatal tests like chorionic villus sampling (CVS) or amniocentesis, and postnatal tests on a baby’s blood or cheek swabs [3]. Note that CVS tests the placenta, which occasionally differs from the baby’s actual makeup (confined placental mosaicism), so amniocentesis may be used for follow-up [4]. Finding an extra chromosome 18 in an appropriate diagnostic sample is strong evidence of trisomy 18.

How the Testing Was Done

Your report will list the specific tissue tested (e.g., amniotic fluid, cord blood, or a vein draw) and the laboratory method [3][4]:

  • Karyotype: A visual picture of the chromosomes under a microscope. It evaluates the total number of chromosomes and large-scale structure [5].
  • Microarray (CMA): A highly detailed scan looking for extra or missing pieces of DNA. It can find small duplications (extra pieces) a karyotype might miss, but generally cannot detect “balanced” structural rearrangements where chromosome pieces trade places evenly without gaining or losing material [6][7].
  • FISH (Fluorescence In Situ Hybridization): A rapid test using glowing tags to count specific chromosomes. FISH is highly targeted—it answers the specific question asked but does not scan the rest of the genome [8]. Follow-up testing is often needed to confirm a rapid FISH result [6].

Decoding the Chromosome Notation (ISCN)

Geneticists use a standardized language, called ISCN, to write results [9]. A typical karyotype result for full trisomy 18 might read 47,XX,+18.

  • 47: The total number of chromosomes found in the cell, instead of the typical 46 [9].
  • XX or XY: Describes the sex-chromosome complement reported by the test (XX or XY) [10][11]. This notation is specific to chromosome patterns and does not describe a person’s complete sex development or gender.
  • +18: Indicates the presence of an extra chromosome 18 [12].

Understanding Cell Counts and Mosaicism

If the report indicates mosaicism, it lists different cell lines separated by a slash (/). For example: mos 47,XX,+18[14]/46,XX[36] [9][3].

  • Cells Examined (Metaphases): The numbers inside brackets, such as [14] and [36], tell you exactly how many cells the lab analyzed [9]. Here, they saw 14 cells with trisomy 18, and 36 typical cells.
  • The “Percentage” Caveat: It is tempting to calculate that 14 out of 50 cells means the baby is “28% mosaic.” However, this percentage only applies to the cells in that specific test tube [13][14]. The proportion of trisomic cells can differ widely between tissues (like blood vs. skin vs. heart), and laboratory culturing can introduce sampling bias [15][3]. Therefore, the percentage in a blood test cannot reliably predict how the baby will be affected or how severe the symptoms will be [9][16].

How the Extra Chromosome is Described

Rather than completely separate categories, your report may use overlapping terms to describe the amount and arrangement of the extra chromosome 18 material [17][1]:

  • Full (Free) Trisomy 18: The extra whole chromosome is present in all or nearly all analyzed cells [17]. This is usually caused by nondisjunction—a random, chance error where chromosomes fail to separate properly during the formation of the egg or sperm [18]. It is not caused by anything the parents did.
  • Mosaic Trisomy 18: Both trisomic and typical cell lines are found in the same individual [17].
  • Partial Trisomy 18: Only a piece (segment) of chromosome 18 is extra [17].
  • Translocation Trisomy 18: The extra chromosome 18 material is physically attached to a different chromosome [1][19].

If a translocation or partial trisomy is found, doctors may recommend individualized testing of the parents’ chromosomes to see if a balanced rearrangement (where a parent has all their genetic material, just rearranged) was inherited [19][20]. This helps determine the risk of it happening in future pregnancies.

What This Report Can and Cannot Tell You

A diagnostic genetic report provides strong evidence of a trisomy 18 diagnosis, which helps your medical team know what to watch for [13]. However, the report alone cannot predict your baby’s exact medical needs [21]. Cytogenetic (chromosome) testing gives a broad overview, but clinical examinations, ultrasounds, and echocardiograms (heart scans) are required to understand your baby’s unique health profile [22][21]. Professional guidance from a medical geneticist or genetic counselor is strongly recommended to translate these laboratory results into a meaningful, individualized care plan.

Common questions in this guide

Does a positive NIPT result prove that my baby has trisomy 18?
No. NIPT, also called cfDNA screening, examines DNA from the placenta in the pregnant parent's blood and estimates the chance of trisomy 18; it is not a diagnosis. A diagnostic test such as amniocentesis or, in some situations, CVS is needed for confirmation.
What does 47,XX,+18 mean on a trisomy 18 report?
It means the tested cell had 47 total chromosomes, an XX sex-chromosome pattern, and an extra chromosome 18. This notation describes a chromosome pattern and does not by itself describe a person's complete sex development or gender. The sample type and the rest of the laboratory result still matter.
Does a 28% mosaicism result mean my baby is 28% affected?
No. If 14 of 50 analyzed cells carry an extra chromosome 18, 28% describes only those cells in that laboratory sample. Different tissues may contain different proportions of cells, and cell culturing can affect the sample, so the number cannot reliably predict your baby's symptoms or medical needs.
How do karyotype and microarray testing differ for trisomy 18?
A karyotype is a microscope-based picture that counts chromosomes and shows large structural changes. A microarray scans DNA for smaller gains or losses that a karyotype may miss, but it generally cannot identify a balanced rearrangement in which DNA is rearranged without being gained or lost.
Why might CVS and amniocentesis show different chromosome results?
CVS tests placental tissue, while amniocentesis tests cells from the amniotic fluid that may better represent the baby's chromosome pattern. Occasionally, the placenta and baby have different cell lines, called confined placental mosaicism, so amniocentesis may be recommended for clarification.
Can a trisomy 18 genetic report predict my baby's exact medical needs?
No. The report provides important chromosome information but cannot predict every health issue or how severely the baby will be affected. Clinical examinations, ultrasounds, and an echocardiogram, along with guidance from a medical geneticist or genetic counselor, are needed to build an individualized care plan.
What does partial or translocation trisomy 18 mean for future pregnancies?
Partial trisomy 18 means only a segment of chromosome 18 is extra, while translocation trisomy 18 means extra chromosome 18 material is attached to another chromosome. Doctors may test both parents' chromosomes for a balanced rearrangement, which can help estimate the chance of recurrence in future pregnancies.

Related questions

How Accurate Is a Positive NIPT for Trisomy 18? Next Steps What Is the Trisomy 18 Recurrence Risk in Pregnancy? What Is the Life Expectancy and Survival Rate for Trisomy 18? Prenatal Trisomy 18 Positive: Why Is Newborn Blood Negative? How Does CVS Compare With Amniocentesis for Trisomy 18?
Show more
How Are Weight Gain and Nutrition Managed in Trisomy 18? What Does Mosaic Trisomy 18 on CVS With Normal Amnio Mean? Why Do Parents Need Karyotype Testing After Trisomy 18? Can Prenatal Ultrasound Predict Trisomy 18 Outcomes? How Common Is Trisomy 18? Pregnancy and Birth Rates What Does Comfort Care Include for Newborns With Trisomy 18? What Happens After a Trisomy 18 Diagnosis at Birth? What Treatments Are Available for Sleep Apnea in Trisomy 18? How Are Breathing Problems Evaluated in Trisomy 18? Can Blood Testing Miss Mosaic Trisomy 18? Other Tissue Tests Trisomy 18: How Are Swallowing and Aspiration Evaluated? What congenital heart defects occur in trisomy 18? Which Newborn Tests Help With Trisomy 18 Comfort Care? Where Is the Best Place to Deliver a Baby With Trisomy 18?

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References

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This page explains trisomy 18 genetic reports for educational purposes and is not medical advice. A medical geneticist or genetic counselor should interpret your baby's results alongside the clinical examination and other tests.