<P> Until recently, the only reliable ways to determine if the fetus has a chromosomal abnormality was to have an invasive test such as amniocentesis or chorionic villus sampling, but such tests carry a risk of causing a miscarriage estimated variously as ranging between 1% or 0.06% . Based on maternal age, some countries offer invasive testing to women over 35; others to the oldest 5% of pregnant women . Most women, especially those with a low risk of having a child with Down syndrome, may wish to avoid the risk to the fetus and the discomfort of invasive testing . In 2011, Sequenom announced the launch of MaterniT21, a non-invasive blood test with a high level of accuracy in detecting Down syndrome (and a handful of other chromosomal abnormalities). As of 2015, there are five commercial versions of this screen (called cell - free fetal DNA screening) available in the United States . </P> <P> Blood testing is also used to look for abnormal levels of alphafetoprotein or hormones . The results of all three factors may indicate a higher risk . If this is the case, the woman may be advised to have a more reliable screen such as cell - free fetal DNA screening or an invasive diagnostic test (such as chorionic villus sampling or amniocentesis). </P> <P> Screening for Down syndrome by a combination of maternal age and thickness of nuchal translucency in the fetus at 11--14 weeks of gestation was introduced in the 1990s . This method identifies about 75% of affected fetuses while screening about 5% of pregnancies . Natural fetal loss after positive diagnosis at 12 weeks is about 30% . </P> <P> Other common chromosomal defects that cause a thicker nuchal translucency are </P>

Normal size of nuchal fold at 13 weeks