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The standard first-trimester screen for Down syndrome, called the combined test, uses three numbers in an algorithm to assess the chance that the fetus has the genetic disorder: markers from the mother’s blood; ultrasound measurements of the fetus’s nuchal fold, an area of tissue at the back of the neck; and the mother’s age.
This last one is perhaps the most well-known risk factor among parents and parents-to-be—it’s common knowledge that as a woman’s age increases, so does her chance of having a baby with a genetic abnormality. Down syndrome occurs when the egg contains an extra copy of chromosome 21. This kind of error is more likely to happen in older eggs, due in part to the decay of proteins within the egg over time.
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Here’s what the algorithm doesn’t account for: Extra genetic material can also attach itself to chromosome 21 in the sperm. Scientists agree that Down syndrome can be attributed to the father in 5 to 10 percent of cases, and some believe that number may be as high as 20 percent. As men age, their risk of fathering a child with Down syndrome may increase—the older the man, the more likely that the process of spermatogenesis, or sperm production will go awry, leading to sperm that contain errors like an extra chromosome. In 2003, a study examining New York State health records found that for parents over 40, paternal contribution to Down syndrome could be as high as 50 percent.
Read full, original post: Down-Syndrome Screening: A One-Parent Test for a Two-Parent Risk
