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Fetal chromosomal aneuploidies can lead to fetal loss and major birth defects. The best known chromosomal aneuploidy is Down syndrome, or medically termed trisomy 21, where the affected individual has an extra full or partial copy of chromosome 21. The extra copy of genetic material is the culprit for causing developmental problem for affected individuals. Three types of Down syndrome exist: trisomy 21 accounts for 95% of cases, translocation accounts for about 4% and mosaicism accounts for about 1%. Less common chromosomal aneuploidies include Edward syndrome (trisomy 18), Patau syndrome (trisomy 13), Turner syndrome (45, X), Klinefelter syndrome (47, XXY), and 47, XYY. Many trisomic fetuses die in utero, and could account for up to 13% of all stillbirths and neonatal death. Even if the fetus would survive, the extra copy of chromosome 21 will affect the baby’s physical and intellectual development into adulthood, causing significant personal, social and economic burden.

The incidence of combined chromosomal abnormalities is as high as one in 160 in the United States or one in 60 in China. Such incidence increases as the mother ages and it could reach as high as 2.5% with maternal age over 35 in China. The estimated incidence of Down syndrome is between 650 and 1000 live births. Such incidence increases significantly as maternal age increases, affecting one in 350