Mosaic Down Syndrome
When a baby is born with Down
syndrome, the healthcare provider takes a blood sample to do a chromosome study. Mosaicism,
or mosaic Down syndrome, is diagnosed when there is a mixture of two types of cells.
have the usual 46 chromosomes, and some have 47. Those cells with 47 chromosomes have
extra chromosome 21.
Mosaicism is usually described as a percentage. Typically, 20 different cells are
analyzed in a chromosome study. A baby would be said to have mosaic Down syndrome
Since the percentage of cells with an extra chromosome is 15 out of 20, the baby would
have a level of mosaicism at 75%. The percentages may vary in different parts of the
body. The percentage of cells in the muscle may differ from the percentage in the
brain, blood, or skin.
Babies born with mosaic Down syndrome
can have the same features and health problems as babies born with trisomy 21 or
translocation Down syndrome, where the extra chromosome material is present in all
cells. But it's possible that these babies may have fewer characteristics of the syndrome
than those with other types of Down syndrome.
Understanding mosaic Down syndrome
There are ongoing studies to find
out if there are any differences in the health of people with mosaic Down syndrome,
depending on how the mosaicism occurred. There have also been studies looking at whether
the percentage of mosaicism can predict the baby’s IQ or whether there will be a heart
defect. These studies show that the percentage of mosaicism doesn’t accurately predict
outcome. Mosaic Down syndrome cases can vary greatly. They can range from having very
mild features to having most of the features of non-mosaic Down syndrome.
What is the chance of having a second child with mosaic Down syndrome?
The risk for chromosome defects in
a pregnancy varies based on the mother’s age at the time of delivery. It increases
yearly with increasing age. But the chance is likely very low.