Prenatal Diagnostic & Screening Testing
What is the difference between a diagnostic and a screening test?
A diagnostic test can usually provide a definitive answer. Generally, it can state whether or not a problem exists. These are usually more than 99% accurate. However, diagnostic tests may have a risk associated with them. An example of two prenatal diagnostic tests are amniocentesis and chorionic villus sampling.
A screening test will not provide a definitive answer. Generally, it will only state if the chance for a problem is increased or decreased. Screening tests usually do not have a risk associated with them. Examples of some prenatal screening tests are nuchal translucency measurement and maternal serum triple screen.
Ultrasound can be a diagnostic tool for some conditions and a screening test for others. When an ultrasound identifies a specific anomaly, such as a heart defect or a spina bifida, it is diagnostic. However, when an ultrasound is used to provide information regarding the risk for chromosome problems, such as Down syndrome, it is a screen.
What is first trimester screening?
The first trimester screen is used to determine if a woman has an increased or decreased chance of carrying a baby with Down syndrome or trisomy 18. It involves an ultrasound examination and a maternal blood sample. The first trimester screen can only be performed between 10 weeks + four days and 13 weeks + three days from the first day of a woman's last menstrual period. The ultrasound is used to measure the fluid that collects behind a baby's neck. This is known as a nuchal translucency measurement.
Babies with excess fluid, or an increased nuchal translucency measurement, are at a higher risk for a chromosome problem, such as Down syndrome or trisomy 18. The second part of the first trimester screen involves taking a sample of the mother's blood to measure two proteins, known as free b -hCG and PAPP-A. These are proteins produced by the pregnancy. Babies with Down syndrome tend to have increased levels of the free b-hCG and decreased levels of PAPP-A. Babies with trisomy 18 tend to have lower levels of both of the proteins.
The first trimester screen is a screening tool, it is not a diagnostic test. Therefore, it can only provide statistical information and not a "yes or no" answer. In other words, the screen may tell you if your baby has an increased or decreased chance of having Down syndrome or trisomy 18, but it cannot tell you whether or not there is a problem. The screen appears to detect about 90% of the babies with Down syndrome and trisomy 18. It is important to remember that a positive or abnormal screen result does not mean that the baby definitely has a problem. Many women with a positive or abnormal screen result have healthy babies.
What is a maternal serum triple screen or alpha-fetoprotein screen?
A maternal serum triple screen is a blood test that is offered to all women under the age of 35 years at the time of delivery. Women 35 and older at the time of delivery may also be offered this screen. This screen is performed between 15 and 20 weeks of pregnancy. It involves the measurement of three chemicals that are found in a pregnant woman's blood stream.
Alpha-fetoprotein or AFP is a protein that is mostly produced by the baby. Babies with a birth defect known as spina bifidia will have increased levels of AFP. The other two chemicals that are measured are human chorionic gonadotropin (hCG) and unconjugated estriol. Babies with Down syndrome tend to make lower levels of the AFP and estriol, but increased levels of hCG. Babies with trisomy 18 tend to make lower levels of all three of these chemicals.
The maternal serum triple screen can detect about 80-85 percent of babies with open spina bifidas and about 60-70 percent of babies with Down syndrome or trisomy 18. A triple screen is considered to be abnormal or "positive" if the risk for any of these conditions is above the cut-off set by the laboratory. A "positive" result does not mean that the baby definitely has a problem. It simply indicates that further testing, such as amniocentesis or ultrasound, may be offered. This means that some women will have a "positive" screen, yet their baby will be fine. Unfortunately, it also means that some women who are carrying babies with one of these problems will have a normal or "negative" screen.
What is amniocentesis?
Amniocentesis is a procedure in which a thin needle is inserted through a woman's belly (not her belly button) into a pocket of fluid that surrounds the baby. The procedure is performed with ultrasound guidance, so the doctor can see the baby and the needle throughout the test. A small sample of amniotic fluid, the fluid in which the baby is floating, is removed.
An amniocentesis can be performed at different times of the pregnancy and for different reasons. An early amniocentesis is done between 12 and 14 weeks, while the traditional timing for the procedure is between 15 and 22 weeks. Generally, amniocentesis performed at these times is done to detect chromosome problems (such as Down syndrome) and open neural tube defects (also known as spina bifida). If indicated, it may also be possible to perform genetic DNA testing on the sample. Amniocentesis can also be performed in the third trimester to determine if the baby's lungs have matured enough to breathe outside the womb. This would be performed if a woman had a complication that may lead to premature delivery or stillbirth.
Amniocentesis has a risk for miscarriage associated with the procedure. An early amniocentesis has a one in 100 (one percent) risk, while the traditional one has a one in 300 (0.3 percent) risk.
What is chorionic villus sampling (CVS)?
CVS is a procedure where a small sample of cells from the baby's placenta are removed. It can be done in two ways, depending on the position of the placenta in the uterus. At WellSpan Maternal-Fetal Medicine, our physicians perform the trans-abdominal procedure, where a thin needle is guided through a woman's belly (not her belly button). If the placenta is at the back of the uterus, a CVS can only be done by inserting a small tube through the woman's cervix. In these cases, the patient may be referred elsewhere. CVS has a risk for miscarriage associated with the procedure of about 1 in 100 (one percent). The procedure is performed between 10 and 12 weeks. Generally, CVS is performed to detect chromosome problems (such as Down syndrome). However, it may also be possible to perform genetic DNA testing if indicated.