It is important to recognize the difference between prenatal screening and testing. Screening gives us a risk calculation. Diagnostic testing gives us a yes or no answer about numeric chromosome problems.
The only way to be certain that an individual does not have a numeric chromosome defect, such as Down syndrome, Patau syndrome or Edwards syndrome, is by means of direct testing of the individual’s DNA. During pregnancy, a DNA sample can be obtained in one of several ways. The most common are chorionic villus sampling (CVS) and amniocentesis. CVS is typically performed between 11 and 13 weeks into the pregnancy, and amniocentesis is usually done between 15 and 22 weeks.
One of our moms, who is a very good poker player, drew the following analogy. The only way to be certain that you have got the winning hand in a poker game is to look at all of your cards, and to look at all of your opponent’s cards. This is similar to amniocentesis or CVS testing.
You can guess at whether you have got a winning hand, though, by a number of means. You can look at the first couple of your cards, and take into account the way that your opponent bets. With each subsequent card, you get a better idea of your hand, and if you are a good poker player, by looking at your opponent’s facial expressions, etc., you can refine your estimate that you have got the winning hand. However, even the best poker players cannot be sure they have got the winning hand without seeing all of their cards and all of their opponent’s cards.
In some families, there is an unusually high risk for chromosome problems based on circumstances in the family. For these families, our genetic counselors can provide guidance about different means of screening and testing.
For families without chromosomal problems, the first way to estimate risk is by means of the mother’s age. Decades ago, only women over 35 were offered testing for chromosome problems. While it is true that women over 35 are at increased risk for chromosome problems, such as Down syndrome, the majority of babies with Down syndrome are born to women under 35, so that even if all women over 35 have chromosome testing done during the pregnancy, we will only detect about a third of babies with Down syndrome before birth.
Over time, we have developed other means of screening and testing for Down syndrome, including integrated screening by means of nuchal translucency measurement, beta hCG and PAPP-A measurement at 11 to 13 weeks and the quad screen at 15 to 22 weeks. Genetic sonography can refine the risk calculations from these tests.
The specifics of integrated screening and genetic sonography are given in separate sections.
After reading these sections, some women will feel satisfied with using screening tests, but others feel the need to know for certain whether the baby does or does not have a numeric chromosomal abnormality, such as Down syndrome. Regardless of Mom’s age, calculation of risk, etc., we offer the option of CVS or amniocentesis to all pregnant women who desire these tests. Depending upon your insurance plan, the testing may or may not be covered.