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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 if:
5 of the 20 cells have the typical number of 46 chromosomes
The other 15 have a total of 47 chromosomes due to an extra chromosome 21
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 the extra chromosome material present in all the cells (nonmosaic). But it's possible that babies with mosaic Down syndrome may have fewer characteristics of the 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 nonmosaic 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.