47,XY,+18: What the Trisomy 18 Karyotype Means
Last updated: October 22, 2026
47,XY,+18
47,XY,+18 is the karyotype of a male with trisomy 18, the cause of Edwards syndrome. Here is what the notation means, how to spot the extra chromosome among the E group, and the related karyotypes to know.
What 47,XY,+18 means
| Part | Meaning |
|---|---|
| 47 | 47 chromosomes in each cell analyzed, one more than the usual 46 |
| XY | One X and one Y chromosome: the person is chromosomally male |
| +18 | An extra whole chromosome 18, giving three copies (trisomy 18) |
The female equivalent is 47,XX,+18. Both are the karyotype of Edwards syndrome caused by a free trisomy.
What to look for in the karyotype
- A count of 47.
- Seven E-group chromosomes instead of six: two 16s, two 17s and three 18s.
- Telling 18 from 17. Chromosome 18 has a very short p arm and a long arm that is mostly dark. Chromosome 17 has a longer p arm, and its long arm has a pale region near the centromere, one dark band farther out and a pale tip.
Check prenatal and newborn cases carefully. Trisomy 18 is often diagnosed from amniotic fluid, chorionic villi or a newborn's blood. Prenatal metaphases are often shorter and at a lower band level, so the 17 versus 18 call relies more on arm ratio and the overall darkness of the long arm.
How trisomy 18 happens
Most trisomy 18 is free trisomy caused by nondisjunction, usually in the mother's egg, and its likelihood rises with maternal age. A smaller share of cases are mosaic, with a mixture of trisomic and normal cells, and a few result from a translocation that adds extra 18 material. Free trisomy 18 is almost always a new event, not inherited.
Related karyotypes
| Karyotype | Meaning |
|---|---|
| 47,XX,+18 | Female with free trisomy 18 |
| mos 47,XY,+18[12]/46,XY[18] | Mosaic trisomy 18: 12 trisomic cells and 18 normal cells |
| 46,XX,del(18)(p11.2) | A different condition: a deletion of the short arm of 18, not a trisomy |
About Edwards syndrome
Edwards syndrome occurs in roughly 1 in 5,000 live births, and many more affected pregnancies end in miscarriage or stillbirth. Babies typically have low birth weight, clenched hands with overlapping fingers, heart defects and other organ differences. It is a severe condition: many affected babies do not survive their first year, although some children live longer, and mosaic cases can be milder.
Common questions
Is 47,XY,+18 the same as Edwards syndrome?
Yes. Full trisomy 18 in all cells is the chromosomal cause of Edwards syndrome.
Why is chromosome 18 hard to identify?
It is a small submetacentric chromosome of similar size to 17. The two are told apart by the length of the short arm and the banding of the long arm, which can be difficult in short or poorly banded metaphases.
Keep learning
This page is for education and training. It is not medical advice and must not be used to diagnose or treat anyone. Laboratory results should be interpreted by qualified professionals following current standards.