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Noninvasive detection of fetal trisomy

Globally, as of 2010, Down syndrome occurs in about 1 per 1000 births[1] and results in about 17,000 deaths.[113] More children are born with Down syndrome in countries where abortion is not allowed and in countries where pregnancy more commonly occurs at a later age.[1] About 1.4 per 1000 live births in the United States[114] and 1.1 per 1000 live births in Norway are affected.[8] In the 1950s, in the United States, it occurred in 2 per 1000 live births with the decrease since then due to prenatal screening and abortions.[71] The number of pregnancies with Down syndrome is more than two times greater with many spontaneously aborting.[9] It is the cause of 8% of all congenital disorders.[1]

Maternal age affects the chances of having a pregnancy with Down syndrome.[3] At age 20, the chance is one in 1441; at age 30, it is one in 959; at age 40, it is one in 84; and at age 50 it is one in 44.[3] Although the probability increases with maternal age, 70% of children with Down syndrome are born to women 35 years of age and younger, because younger people have more children.[3] The father’s older age is also a risk factor in women older than 35, but not in women younger than 35, and may partly explain the increase in risk as women age.[115]