Genetic screening

All pregnant women, regardless of age, are offered genetic screening for Down Syndrome (Trisomy 21), Trisomy 18 (a rare chromosomal abnormality) and neural tube defects. Approximately 1 in 750 babies are born with Down Syndrome. The risk of having a baby with Down syndrome increases with maternal age. The choice of screening test depends on many factors, such as gestational age at your first appointment, number of babies, previous obstetrical history, test detection rates, and risks of an invasive procedure. The decision for a genetic screening test versus a diagnostic test is a personal one. Patients will weigh the benefits and risks of screening versus diagnosis differently. Some patients may elect to decline any testing, both screening and diagnostic simply because they would not use the information to make any further management decisions.

What are your age related risks for Down syndrome and other chromosomal abnormalities?

If it is determined that your baby has a chromosomal abnormality, unfortunately there is no medical intervention that can be done to change the diagnosis. As you consider whether or not you would choose to pursue screening or diagnostic test, ask yourself the following questions:

• If you had a positive screening result, would you choose to pursue diagnostic testing to find out definitively whether or not your baby had a chromosomal abnormality? Diagnostic tests include chorionic villus sampling and amniocentesis (discussed below).
• Would you use the information to prepare yourself for having a child with a chromosomal abnormality?
• Would you consider terminating the pregnancy if you knew that your child had a chromosomal abnormality?

There are two major types of tests available: screening tests and diagnostic tests.

Screening tests will assess the risk of your baby having Trisomy 21 and Trisomy 18. It will not detect all genetic abnormalities. These tests are not considered diagnostic tests (a definitive test). Genetic screening tests are risk assessments of what your baby’s chance is of having a genetic abnormality. Screening tests are statistical models of assessing risk and therefore are associated with false positive results and false negative results. The two screening options offered are: the sequential screen and the quad screen.

A negative screen means that most likely your baby will not have a genetic syndrome. However, a negative screen does not guarantee that the baby does not have a genetic abnormality or another type of birth defect. It does lower the risk of your baby having an abnormality. A positive screen means that your baby may have an increased risk of chromosomal abnormality. It does not mean that your baby has a genetic syndrome. In fact, in the majority of cases, there will be no abnormality.

If you do have a positive screen, you will be given a numerical risk assessment. This will allow you to compare your age related risk versus your screen risk. For example, if you are 35 years old, your risk of having a baby with Down Syndrome based on your age is 1 in 270. After the screening test, your risk may be in 1 in 5,000 or 1 in 20. If your screen risk is elevated, you will be offered a targeted ultrasound to evaluate for any possible markers of chromosomal abnormalities and/or diagnostic testing via a chorionic villus sampling or amniocentesis (discussed below).

The sequential screen is performed between 12 ½ and 13 ½ weeks gestation. It is a combination of maternal blood serum markers and an ultrasound that measures the baby’s nuchal translucency (the skin thickness on the back of the neck). These initial results are typically available 5 – 7 days after the test is performed. These results are then combined with another set of maternal blood markers drawn around 16 weeks gestation. The detection rate for Trisomy 21 is 90 %. The detection rate for Trisomy 18 is 90%. The sequential screen has a 5% false positive rate.

The second trimester screen or quad screen involves only a maternal hormone evaluation via a simple blood draw. This is typically done at your 16 week visit. The quad screen has an 81% detection rate for Down Syndrome with a 5 - 10% false positive rate.

A key difference between the sequential screen and the quad screen, apart from the detection rates, is the timing of the results. The preliminary results of the sequential screen are available around 13 – 14 weeks, whereas the quad screen results are available at 16- 17 weeks. Receiving the screening results early would allow you to pursue definitive diagnosis via a CVS and potentially pursue a termination of pregnancy, if you choose earlier than if the quad screen had been done.

All patients are offered a second trimester anatomy scan at 18-20 weeks gestation. This ultrasound does not evaluate the baby’s genetics and is not a substitute for genetic screening. To illustrate this point, 40% of babies born with Down Syndrome will appear normal on their 20 week ultrasound.

The diagnostic tests offered are chorionic villus sampling and amniocentesis. These procedures will offer definitive diagnosis of any chromosomal abnormality as it maps your baby’s entire chromosomal pattern. Both procedures are performed by perinatologists. Chorionic villus sampling (CVS) is performed in the first trimester. A small amount of the placenta is removed with a needle being passed through your cervix or through your abdominal wall. Amniocentesis is performed between 16 and 22 weeks. It involves withdrawing a small amount of amniotic fluid by passing a needle through your abdominal wall. As both of these tests are invasive, they have inherent risks of pregnancy complications including: infection, premature rupture of membranes and miscarriage. The pregnancy complication rate for CVS is approximately 1 per 150 -200 pregnancies and for amniocentesis 1 per 300-500 pregnancies.