Odds of Down Syndrome by Maternal Age and Gestational Age

The odds of any given fetus, near the end of the first trimester, having three chromosomes at the 21st site (trisomy-21) and thus having Down syndrome are shown in the 10-weeks column. These data were collected before widespread prenatal testing and induced abortion. As you see, almost half of all Down syndrome fetuses are spontaneously aborted. Also note that the odds of Down syndrome increase steadily with the age of the mother. Nonetheless, even for women giving birth in their 40s, most newborns do not have chromosomal abnormalities.