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. 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. 

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 a triple screen blood test 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 triple screen 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 triple screen and ultrasound results. 

Some of the centers in Europe are not using serum screening because they do not think it is helpful. The fetal heart rate at the time of nuchal translucency screening is being used by some centers. The fetal heart rate tends to be elevated in Down syndrome fetuses. 

And remember, as always, it makes a difference who performs your ultrasound exam.

Prenatal Diagnosis Of Down Syndrome

Prenatal diagnosis means trying to discover what medical problems a fetus has while it is still within the uterus.
At the present time in the United States, the Triple Screen blood test is probably offered to most pregnant women to assess their risk of having a baby with Down syndrome or neural tube defects (spina bifida). This method of screening can lead to the diagnosis of about 65 % of the Down babies before they are born. 

The risk of having a Down baby increases with the mother’s age, but because most babies are born to young women, most Down babies are born to young mothers. Therefore all pregnant women, young and old, should be screened. 

Most women in the United States have at least one ultrasound during their pregnancy, and of course ultrasound is another important method of prenatal diagnosis. 

The diagnosis of Down syndrome and other chromosomal abnormalities is not made by triple screen, but rather must be made by amniocentesis or chorionic villus sample (CVS). The triple screen blood test and ultrasound help us to decide who should have amniocentesis or CVS. 

At the present time in the United States the diagnosis of Down syndrome is most often made in the second trimester of pregnancy. 

It is now possible to assess risk of Down syndrome and other chromosomal abnormalities (Trisomy 18, trisomy 13, and Turner’s syndrome) in the first trimester of pregnancy. This can be done by nuchal translucency done between 11 weeks 0 days and 13 weeks 6 days from the last menstrual period. Also a blood test done at the same time, plus factoring in the maternal age, can lead to the identification of about 90% of Down babies. 

Earlier identification of a baby with a chromosomal abnormality means a safer termination if that is elected, more time to prepare for the birth of a Down baby, more time to seek consultation. 

The nuchal translucency thickness may be increased not only in babies with chromosomal abnormalities, but also in babies with major heart defects, and many other anomalies. 

The first trimester risk assessment does not include neural tube defects, so it is still necessary to have an alpha fetal protein (AFP) test at about 16 weeks gestational age to assess risk of neural tube defects. A carefully done ultrasound at about 18 to 24 weeks may also rule out neural tube defects. 

During the early ultrasound for nuchal translucency, at around 12 weeks, (11 weeks 0 days to 13 weeks 6 days to be exact) it is possible to date the pregnancy quite accurately, diagnose twins, and sometimes see major problems. 

At the conclusion of this early assessment, the mother is given a risk of her having a baby with Down syndrome. This risk is given in the form of a ratio (1 chance in 250, for example, expressed as 1/250). 

We start with a risk based on the maternal age, and the gestational age (the risk of Down syndrome actually decreases as the pregnancy progresses, because many such pregnancies end in miscarriage). The results of the nuchal translucency measurement and the results of the blood tests then modify that risk. 

Nuchal translucency measurement is not the standard of care in the United States. There is a study being done in the USA called the FASTER study, to evaluate the value of nuchal translucency. FASTER stands for “First And Second Trimester Evaluation of Risk”. This study is funded by the National Institutes of Health. There are ten medical centers throughout the United States participating in the study. 
The primary goal of the FASTER study is to compare first trimester screening (nuchal translucency and maternal serum PAPP-A and HCG measurements) with second trimester maternal serum screening.

Excerpt from the Medical Ultrasound Fact Sheet from the American Institute of Ultrasound in Medicine (AIUM)