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. Some have the usual 46 chromosomes, and some have
47. Those cells with 47 chromosomes have an 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 the 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