Prenatal Genetic Testing
During pregnancy, there are many different screening and diagnostic tests that your doctor uses to monitor your health and the health of your baby. Screening and testing for genetic problems are also offered to pregnant mothers to assess the fetus's wellbeing and to look for birth defects or other inherited and genetic conditions such as Down syndrome (DS), trisomy 18 (an extra 18 chromosome) or neural tube defects (spina bifida - an opening in the spine).
There are 2 types of prenatal tests: screening tests - these tests determine if the fetus has an increased risk (chance) that there may be a genetic condition and diagnostic tests - such as amniocentesis and chorionic villus sampling (CVS) - that give a specific diagnosis.
Prenatal Screening Tests
First Trimester
First check: This test is done between 11 and 14 weeks of pregnancy and includes both a blood test and an ultrasound examination. This test screens for Down syndrome and trisomy 18. It cannot screen for neural tube defects (spina bifida).
The blood test measures 2 substances:
- Human chorionic gonadotropin (hCG) - a hormone that is produced by the placenta
- Pregnancy associated plasma protein (PAPP-A) – a protein produced by the placenta
The ultrasound exam looks specifically at the fetus's neck and measures the space between the skin and the tissue that covers the top of the spine. This is called nuchal translucency. The ultrasound must be done by someone that has been certified to do this procedure.
The results: If the blood test shows too much or not enough of the 2 substances measured and/or the nuchal translucency is "thicken", this may indicate that the fetus has an increased chance of having DS or trisomy 18.
Second Trimester
Quad test: This test is a blood test that is done in the 2nd trimester - between 15 and 23 weeks of pregnancy. It measures 4 different substances that can tell you if you have an increased chance of having a baby with Down syndrome, trisomy 18 or spina bifida. The test measures 4 substances:
- Human chorionic gonadotropin (hCG) - a hormone that is produced by the placenta
- alpha-fetoprotein (AFP) - a substance that is made by the fetus's liver
- Estriol (uE3) - a substance that is produced by the placenta
- Inhibin A - a protein made by the placenta
The results: Results from the quad test are combined with other factors such as the woman's age, weight, race and chronic diseases such as diabetes to come up with an estimate of risk or the chance that the fetus could have Down syndrome, trisomy 18 or spina bifida.
Remember, these screening tests can only tell you about your potential risk to have a baby with Down syndrome or problems such as trisomy 18 or spina bifida - they cannot tell you for sure that the baby is affected. A screening test can be abnormal and there is no problem with the fetus. This is the case most of the time. And, because prenatal screening and diagnostic tests only test for some birth defects and genetic conditions, there is still a chance that the baby can be born with a genetic disorder.
Prenatal Diagnostic Tests
First Trimester
Chorionic villus sampling (CVS) is done between 10 and 12 weeks of pregnancy. A very thin flexible tube can be inserted through the abdomen or through the cervix and some of the tissue surrounding the placenta and fetus can be collected to look at the chromosomes.
Second Trimester
Amniocentesis is usually done between 15 and 22 weeks of pregnancy. A very thin needle is inserted into the amniotic sac through the abdomen and a small amount of the fluid that surrounds the fetus is collected. This fluid has cells from the fetus and a chromosome analysis can be done. The fluid can also be tested for spina bifida.
Because both CVS and amniocentesis are invasive diagnostic tests, there is a chance of complications that can lead to a miscarriage following either test. For amniocentesis, doctors usually say the risk is about a half of a percent (0.5% or 1 in 200), but a recent study suggests that the risk of miscarriage may be less than this, about 0.06% or 1 in 1600 pregnancies. The risk for miscarriage is a bit higher for CVS, about 1% (1 in 100).
Comprehensive Ultrasound: Ultrasound (US) uses sound waves to look for structural defects (major birth defects) such as a heart defect or cleft lip and palate. US also will look for "markers" that are associated with Down syndrome. Markers are minor findings that are not abnormalities in themselves, but may be clues that the fetus could have Down syndrome. An US is an appropriate follow-up evaluation to an abnormal maternal serum screen.
Who should consider having a prenatal genetic diagnostic test? Any of the following women may want to consider having prenatal genetic testing done:
- Mothers older than 35 years old
- Mothers whose ethnic background has been associated with certain types of genetic disorders such as Tay Sachs disease (Ashkenzi Jewish background), sickle cell anemia (African American background) or cystic fibrosis (Caucasian background).
- Mothers who have a family history or who had a previous child with a genetic condition or birth defect that can be diagnosed during pregnancy.
- Mothers who had a prenatal screening test that showed they were at increased risk of having a baby with a chromosomal abnormality such as DS, trisomy 18 or spina bifida.
Remember, whether you decide to have a prenatal screening test or diagnostic test done, it is your decision to make. Your doctor should talk to you about what test options are available, what the tests can tell you and their advantages and disadvantages. Your doctor may also refer you to a genetic counselor to discuss both prenatal screening tests as well as diagnostic tests. This is an area in which they have a lot of expertise. You can locate a genetics center near you through the National Society of Genetic Counselors Resource Center website (http://www.nsgc.org/resourcelink.cfm).