There are several tests available to pregnant women that can detect genetic and development errors in the growing fetus. The screening tests are generally very good at detecting fetuses with the targeted problems and will accurately identify about 80-95% of them depending on the testing combination. While most of the tests are available to all women, if you will be 35 or older when you give birth or if you have had a previous baby with a genetic disorder, initial tests may be skipped altogether.
The disorders most commonly screened for are:
The initial tests are screening tests only. This means that they only predict an increased risk and cannot tell you for sure whether your fetus has the disorder. In fact, out of 100 women who screen positive, about 95 will have a normal fetus.
Further tests are needed to give a final answer. Some of the tests, such as amniocentesis or chorionic villi sampling, carry a real risk of ending the pregnancy because they require sampling of actual amniotic fluid or placenta tissue. Ultrasound and non-invasive prenatal testing are able to rule out some of the problems but are still not able to give a definite answer. Which tests you choose or if you choose no testing at all will depend a lot on what you would do if you discovered a potential problem with your baby.
Some women will choose to test because they like to know before the baby is born if there will be problems. This allows them to prepare and access the resources they will need. Some women would choose to terminate pregnancy, particularly one where the condition is not compatible with life. For other women, the risks associated with diagnostic testing are not acceptable and they do not want the uncertainty of a positive screening test alone so they choose no testing at all. For other women, the results of the test would not change their pregnancy choices.
There is not a right or a wrong answer about testing. The choice is entirely up to you and this information is presented as information only and not an opinion or judgement on what any pregnant woman should choose. Remember to check with your insurance company first to see what restrictions they may have for fetal testing. Some tests may not be covered unless you are at increased risk.
Because the testing can be complex, we usually arrange a visit with a genetic counselor to discuss your options and to arrange testing. This is true for all tests except the quad screen, which we order directly.
Following is a very brief description of the most commonly used screening tests. Different combinations will be used depending on whether accuracy or early detection is most important.
Non-Invasive Prenatal Testing (NIPT): This brand new test allows examination of the fetal DNA taken from the mother's blood. This has the potential to be very accurate without putting the fetus or mother at risk from the testing. It can be done as early as 9-10 weeks of gestation. Currently it is only recommended for certain high risk pregnancies including mothers over 35. This will not detect neural tube defects such as spina bifida.
betaHCG: While usually used for confirming pregnancy, this blood test can also be used to indicate a risk for Down Syndrome, Trisomy 18, molar pregnancy, multiples or an increased risk of stillbirth. This is a part of the second trimester blood test known as the "Quad" screen. If it is done in the first trimester, it is usually combined with a specialized ultrasound called a nuchal translucency measurement for Down Syndrome.
PAPP-A (Pregnancy Associated Plasma Protein A): This is decreased in Down Syndrome, Trisomy 13 and Trisomy 18. It may also predict a small for gestational age baby or stillbirth. Very high levels may predict a large for gestational age baby. This may be done in the first trimester along with a nuchal translucency measurement.
Nuchal Translucency Measurement (NT): This is a specialized ultrasound done between 11 and 14 weeks that can indicate an increased risk for Down Syndrome. This is only done by specialized sonographers and in our area is arranged by the genetic counselors through Maternal-Fetal Medicine either at the University of Utah or Intermountain Medical Center.
Quad Screen: This is a blood test that can be done between 15 and 22 weeks that is composed of 4 different tests. It includes maternal serum alpha-fetoprotein, maternal serum estriol, maternal beta hcg, and inhibin A. Combinations of high, low and normal levels are used to calculate a risk profile for trisomy 13, 18, 21 and neural tube defect. These risk profiles are different depending on race, age, weight and whether you are diabetic. If our calculations of the age of the baby are off, it can significantly decrease the accuracy of the test.This is the only testing we order through our office.
Maternal serum alpha-fetoprotein: This is the test for spina bifida and cannot be done before 15 weeks. It is part of the Quad Screen and other integrated screening, but may be done separately.
These next two tests are diagnostic tests which are able to give very accurate results. Because they require tissue or fluid sample from the baby, they carry a risk of disrupting the pregnancy and can cause a miscarriage.
Chorionic Villus Sampling (CVS): This test involves removal of a very small amount of the placental tissue and can be done in the first trimester. Because a greater concentration of fetal DNA is collected, results are generally available in less than a week.
Amniocentesis: This test is done in the second trimester using a long needle to remove about 1 tablespoon of fluid. The cells are then grown for about 2 weeks until the chromosomes can be examined for defects. Complications from this procedure can include cramping, bleeding, accidental rupture of the membranes, infection and miscarriage. The risks seem to be less when the procedure is preformed by a highly skilled physician. Generally quoted rates for fetal loss range from 0.5%-1%.
The following sites may be useful as you research this topic.
American Academy of Family Physicians Guidelines
Overview of prenatal screening
Description of risk and testing from the UofU
ACOG opinion on the Non-invasive fetal DNA test
Mayo Clinic overview on non-invasive fetal DNA testing
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