Blog 115 B – Tests and more Tests
This blog should help you work through and understand the various screening test options for you and your baby. This may help you decide if you want any or all of the tests done. We’ll start with the earliest tests and end with the more “invasive” types of tests.
FIRST TRIMESTER SCREENING TESTS
As you may recall, a pregnancy is divided up into 3 sections called trimesters. At the end of the very first trimester (between the 11th and 14th week) two screening tests are offered. They check for the mom’s risk of having a baby with chromosome problems such as Down syndrome or trisomy 18.
There are 2 parts to the test. They include having some blood drawn (from the mom, not the baby) and a special ultrasound looking at the back of the neck of the baby and measuring what is called the “nuchal translucency”
The blood test measures two hormones in the mom’s blood. They are:
● Pregnancy-associated plasma protein-A (PAPP-A) – This stuff is made from the covering of the fertilized egg. In a healthy/normal pregnancy the level should gradually get higher and higher. If the level is low (doesn’t get higher) then that is associated an increased risk of having a baby with a chromosome problem such as Down syndrome or trisomy 18.
● Human chorionic gonadotropin (hCG) – This stuff is made by the placenta. If a portion of this hormone is too high, there is an increased risk of having a baby’ with Down syndrome or trisomy 18.
The very early special ultrasound of the baby’s neck is called a nuchal translucency exam. The thickness in the back of the baby’s neck is measured. If it is thicker than normal, it may be a sign of Down syndrome, trisomy 18, a heart defect, or other problems with the baby’s chromosomes.
If you choose to have these tests, the results will estimate your chance of having a baby with a chromosome problem. These tests do not diagnose chromosome problems and they are not 100% accurate. You will be told you either “screened negative” or you “screened positive”.
“Screened negative” means your chance of having a baby with one of these problems is very low. At that point your testing is done.
“Screened positive” means your chance of having a baby with a chromosome problem is higher than normal for someone your age. It does not mean your baby has a birth defect. At that point you will be offered some more advanced tests (like an amniocentesis). These tests check the baby’s chromosomes and will give more information and help diagnose a possible problem.
TAKE NOTE: Most moms who “screen positive” and have advanced testing find out they have normal healthy babies.
“Screen negative” results mean that your chance to have a fetus with these disorders is low and no follow-up testing is offered. It is important to remember that a “screen negative” result does not guarantee that there are no birth defects. If you have a “screen negative” result, you will not be offered follow-up diagnostic testing such as CVS or amniocentesis. You will also be offered another serum screening test, done in the second trimester, that measures a substance called AFP. The amount of AFP present screens for open neural tube defects such as anencephaly and spina bifida.
SECOND TRIMESTER SCREENING TESTS
If you started your prenatal care after your 14th week (and before your 21st week), or you declined the early screening tests, you will be offered another screening blood test. This test, done between 15-21 weeks, screens for Down syndrome, trisomy 18, and for defects in the baby’s spinal cord and brain (called neural tube defects).
What screening tests are done in the second trimester?
In the second trimester, a test called “multiple marker screening” is offered to screen for Down syndrome, trisomy 18, and neural tube defects. This test measures the level of three or four of the following substances in your blood:
• Alpha-fetoprotein (AFP)—A substance made by a growing fetus, which is found in amniotic fluid, fetal blood, and, in smaller amounts, in the mother’s blood.
• Estriol—A hormone made by the placenta and the liver of the fetus.
• Human chorionic gonadotropin—A hormone made by the placenta.
• Inhibin-A—A hormone produced by the placenta.
The test using the first three of these substances is called a triple screen. When the fourth substance (inhibin-A) is added, the test is called a quadruple screen. The triple screen test detects Down syndrome in 69% of the cases. The quadruple screen detects Down syndrome in 81% of the cases. The AFP test detects neural tube defects in 80% of the cases. These tests usually are done around 15-20 weeks of pregnancy. The stage of pregnancy at the time of the test is important because levels of the substances measured change during pregnancy.
What is combined screening?
The results from both first- and second-trimester tests can be combined to increase their ability to detect Down syndrome. When both the first- and second-trimester tests are used, about 90-95% of Down syndrome cases can be detected. With this type of testing, the final result may not be available until all tests are completed.
What happens when the results from screening tests are a cause for concern?
If the results of a screening test or other factors raise concerns about your pregnancy, diagnostic tests can be done to provide more information. These tests include the following:
• Detailed ultrasound exam—A type of ultrasound exam that can help explain abnormal results and provide more detailed information about the growth and development of the fetus.
• Amniocentesis—A procedure in which a small amount of amniotic fluid and cells are withdrawn from the sac surrounding the fetus and tested.
• Chorionic villus sampling (CVS)—A procedure in which a small sample of cells from the placenta is tested.
A “screening” test that is becoming very popular and decreasing patients desires to do invasive testing is called NIPT( non invasive prenatal testing) Different labs use different methods but essentially are assessing BABY’S DNA in mom’s blood. Depending on the condition and the lab there is a very high detection rate. This technology deserves its own blog.
Let us know if you would like a blog on NIPT!!
thanks for reading
Dr Betoni and Camilla C-NP.
Anencephaly: A type of neural tube defect that occurs when the fetus’s head and brain do not develop normally.
Human Chorionic Gonadotropin (hCG): A hormone produced during pregnancy; its detection is the basis for most pregnancy tests.
Nuchal Translucency Screening: A special ultrasound test of the fetus to screen for the risk of Down syndrome and other birth defects.
Placenta: Tissue that provides nourishment to and takes waste away from the fetus.
Ultrasound Exam: A test in which sound waves are used to examine internal structures. During pregnancy, it can be used to examine the fetus.