All women should be offered prenatal genetic screening. Many women choose to have prenatal genetic screening; it is up to you whether you would like these tests. The results of genetic screening can provide important information, but also can lead to difficult choices. If the tests showed your baby had a serious condition, would you continue the pregnancy? Would you want to know this information so you can prepare for the birth of a baby that might need special care? These are difficult and very personal decisions. No matter what your choices are, your health care provider will support you throughout the process.
There are two main types of blood tests. The standard prenatal blood testing measures chemicals in the blood that indicate the relative likelihood of abnormalities. More recently, tests have become available that are able to detect the baby’s DNA in the mother’s blood. Whichever test you have, it is still a screening test. Such a test cannot definitively establish that an abnormality is present, or absent. These tests only estimate chances. Abnormal results require further testing for diagnosis or reassurance.
There are several different types of prenatal genetic screening tests. Which types are available to you depend on where you live and how far along you are in your pregnancy. If you choose to have genetic screening, your ultrasound(s) (if done) and a blood sample or samples will be used to test for signs of certain conditions. These include genetic abnormalities (e.g., Trisomy 18, Down syndrome) and neural tube defects (e.g., spina bifida). The results of prenatal screening tests are not a diagnosis. They provide you with a likelihood that your baby has one of these conditions. There are both false positive and false negative results, so a positive screen does not mean your baby is affected. For proper diagnosis, you will need to undergo further evaluation, such as non-invasive prenatal testing (which is also a screening test, but with greater precision), or invasive testing such as amniocentesis or chorionic villus sampling. These two invasive procedures (described below) can determine whether a disease is present or not, and not just the chance of it. Because they are “invasive” and require putting an instrument into the uterus to obtain fetal cells, there is a risk of pregnancy loss.
Non-invasive prenatal testing (NIPT) is a relatively new, highly effective method of screening for Down syndrome and other similar genetic abnormalities. It is a blood test that measures cell-free fetal DNA that is present in the mother’s blood. It can also be used to determine fetal sex, identify the presence of an Rh-positive fetus in an Rh-negative mother, and to determine some genetic abnormalities that are passed from the father’s genes. There is a possibility of a failed test (no results), a false negative, or false positive result. The test is not definitive, and all positive screening results will need to be confirmed with more invasive fetal diagnostic testing before any decisions are made.
In this test, a small amount of amniotic fluid will be taken out of the uterus with a needle inserted through your abdomen. The doctor will use ultrasound to guide the needle to an area far from the baby. Some of the baby’s cells will be present in the fluid. These are extracted and tested for genetic abnormalities. This test has a low risk of miscarriage of 0.5% -1%. If you have a fetus diagnosed with a genetic abnormality, you have the choice of continuing or terminating the pregnancy.
Chorionic villus sampling is a prenatal test where a tiny piece of placental tissue is taken either through the cervix or the abdomen. The sample is then tested for genetic abnormalities. This test can be performed earlier than amniocentesis, usually between 10 and 13 weeks. It also carries a small risk of miscarriage of 1-2%. The risk is a little higher than amniocentesis, but the benefit is that a diagnosis can be made earlier in pregnancy.
Nuchal translucency is a collection of fluid under the skin of all baby’s necks. The test measures the thickness of the fluid. If it is thicker than normal, it means that the baby could have Down syndrome or other genetic abnormalities. This test is also not a diagnosis, but gives a measure of risk.