Chromosomal Screening - MacArthur Medical Center

Abnormal Chromosomal Screening: Will My Baby Have Down Syndrome?

With so many technological breakthroughs recently, the field of prenatal diagnosis has undergone major advances. Over the last 30 years, detection rates for Down syndrome and other chromosomal abnormalities (Trisomy 18 and 13) have increased from a mere 60% to well over 95%. Chromosomal abnormalities were once thought of as being an issue primarily for “older” women (pregnant women over the age of 35), but now with the ability to detect these problems so accurately, ACOG (American Congress of Obstetrics and Gynecology) now recommends that ALL pregnant women who present for prenatal care at less than 20 weeks be offered some form of prenatal screening for chromosome problems such as Down syndrome.

Discussing the topic of chromosome screening with patients can cause confusion and anxiety. Many patients wonder why this testing is needed, how testing is done, at what gestational age can testing be done, and of course, the big question: WHAT HAPPENS IF MY SCREENING TEST IS ABNORMAL? These are all great questions that are difficult to explain during one quick office visit. The following questions/answers will hopefully clear up any myths or rumors that you might be hearing about chromosomal screening.

What is aneuploidy?

Aneuploidy means abnormal chromosome number (either missing or extra chromosomes). Aneuploidy results from a problem very early in pregnancy when chromosomes fail to separate properly and therefore an “extra” chromosome is obtained.  Most commonly this occurs on chromosome numbers 21 (Down syndrome), 18 (Edwards syndrome), and 13 (Patau syndrome). When a fetus has an extra chromosome at one of these positions, the outcome can have varying degrees of abnormalities. These range from learning difficulties, short stature, or distinct facial features to even more serious problems such as structural birth defects (heart, kidney, GI tract) or a baby who will be unlikely to survive after birth.

What is a screening test?

A screening test is a test that is performed to give information about a patient’s risk of having a baby with certain birth defects/chromosomal problems. It DOES NOT have the ability to diagnose a chromosomal problem. Instead, it simply indicates that a patient is at higher risk and needs more invasive testing to confirm the screening test results. The higher the detection rate, the better its ability to pick up the abnormality, and therefore the better the screening test.  Unfortunately, screening tests cannot detect 100% of all abnormalities and rarely even give false positive results.

How is a chromosomal screening test performed?

Levels of certain hormones/proteins are often abnormally high or low with certain chromosomal problems. These levels can be measured by simply analyzing a mother’s blood during specific times in pregnancy.

What specific tests are involved with chromosomal screening?

There are many different tests available to screen for aneuploidy. It’s more important that testing IS BEING DONE rather than WHICH test is being performed. With that said, different tests have different detection rates and which test you receive will depend on your doctor’s office’s resources and laboratory. At MacArthur OBGYN, you will have your blood tested on two separate occasions and an ultrasound to evaluate the neck of your baby (nuchal translucency) as babies with thicker neck measurements on ultrasound are at higher risk for having chromosome problems.

When will I be screened for chromosomal problems?

Typically, we at MacArthur OB/GYN use an “integrated” screening test. This involves combining the results of a test from 11-14 weeks with another test after 15 weeks. During your visit between 11 and 14 weeks, your blood will be drawn and an ultrasound will be done to measure the neck of your baby. These tests are combined with another blood test after 15 weeks to calculate a combined risk. The result of the test does not come back until after the second blood test is performed after 15 weeks. The combination of both of these tests (as opposed to only one test) leads to a higher detection rate and more accurate results. If for some reason a pregnant woman misses her first screening test from 11-14 weeks, but is still less than 23 weeks, a single blood test called the Penta screen can be performed to determine her risk. The Penta screen has a lower detection rate than traditional integrated screening but is still effective.

How accurate are chromosomal screening tests?

The detection rate of the integrated screening test is approximately 94-96%, while the Penta screen (the testing that occurs only in the second trimester after 15 weeks) has a detection rate of approximately 85%.

What happens if I have a positive screening test result?

If you have a positive screening test, first of all, DO NOT PANIC. You simply are deemed higher risk. It DOES NOT mean that your baby has a chromosome defect. A positive screening test will typically be followed by a referral to a Maternal-Fetal Medicine specialist (high risk pregnancy doctor), where he or she will offer the following:

  1. A targeted ultrasound to look closely at the other parts of the baby (heart, stomach, face, spine, kidneys) to see if there are any obvious birth defects that suggest chromosomal problems or other genetic syndromes.
  2. A confirmatory test to diagnose what type, if any, chromosome abnormality is present.

Confirmation of aneuploidy can be done in several different ways depending on the doctor.  Some will offer you an amniocentesis. This is a procedure in which amniotic fluid (which has fetal cells) is drawn from within the amniotic sac and sent for chromosomal analysis. Others will offer a chorionic villus sampling (CVS), in which a small sample of cells from the placenta is tested. Other testing that can be done are maternal blood tests such as the Panorama, Harmony, or Materni21 tests, which detect cell free fetal DNA in the maternal circulation. This testing involves new technology and is indicated for pregnant women over 35 years old or for women who have an abnormal standard screening test or ultrasound findings suggestive of aneuploidy. These tests are not confirmatory tests but do have detection rates up to 99% and are therefore very helpful to patients who either decline amniocentesis or where amniocentesis is not feasible.

With new technologies we now know even more about the health of babies before they are born. Improved ultrasound quality and more accurate blood testing has given us more insight into a baby’s genetic makeup, specific syndromes, or any structural problems that he or she may have. Chromosomal screening is an integral part of prenatal care and hopefully the information above has helped make it a less intimidating and confusing topic.

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