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Genetic Ultrasound Screening

Another area that is receiving a great deal of attention is genetic ultrasound screening. There is really nothing special about a genetic prenatal ultrasound exam: it is just a very good, complete prenatal scan to include careful views of the heart and the outflow tracts of the heart. This is of particular importance to women who will be between 35 and 40 at the time of delivery. As you may know, the risk of Down syndrome and other chromosomal abnormalities increases with age. The present standard of care is to offer amniocentesis to all women who will be 35 or more at the time of delivery.

There is a small risk to the amniocentesis. Somewhere between 1 in 200 to one in 300 cases will have a significant problem from the amniocentesis itself (although in the recent FASTER study, mentioned elsewhere in this website, the risk of loss of the baby from amniocentesis was 1 in 1700).

The age of 35 years is selected because at that age your risk of Down syndrome or other chromosomal abnormality at least equals, and often exceeds the risk of the amniocentesis. It does not make sense to have a procedure with a risk of 1:200, to discover if you have a Down baby, when your risk of having a Down baby based on your age is 1:2000. However, amniocentesis (and chorionic villus sampling, done earlier in pregnancy) will give you a positive diagnosis: your baby does or does not have Down syndrome. The screening techniques can never tell you absolutely that your baby is normal, but can only give you a risk figure.

If you are between 35 and 39 years of age, if you have first or second trimester screening that indicates a low risk for Down, and if your ultrasound is entirely negative, then your risk of having a baby with Down is considerably reduced. It would be reasonable not to have an amniocentesis. Or to put it another way: if your blood tests and ultrasound are negative, then the risk of the amniocentesis is probably greater than your risk of having a baby with Down syndrome.

However, if you will be aged 40 or more years at the time of delivery, you should have amniocentesis regardless of your blood screen and ultrasound results.