What is Prenatal Screening?
Prenatal screening tests are blood tests that look for only certain birth defects that can happen to anyone. There are different types of screening tests; some can only be done during certain times of your pregnancy. These screening tests have no risk of harming you or your baby.
Is Prenatal Screening Right for Me?
It is your personal choice to have prenatal screening tests. Prenatal screening is not for everyone. Positive results on genetic screening tests do not mean the baby has the condition. Positive results mean the chance the baby has the condition is higher than average. Here are some questions to consider before you decide to have prenatal screening:
- How would you feel if the screening results were positive?
- Would you consider a diagnostic test if a screening test were positive?
- If not, would you be ok waiting until the baby is born to find out for sure if the condition is present?
- Do you think this information would help you feel more prepared?
- Does the possibility of the results being uncertain make you anxious?
It is important to keep in mind that these tests are not appropriate for all women. Different tests might be better for women at a higher risk due to their age, family history or health history. These tests do not screen for all genetic conditions or birth defects. Each screening test has different accuracy for detecting certain conditions. Please speak with your doctor to help you decide which test is most appropriate for you. If you receive a positive, or abnormal, result from any of these screening tests, you should see a genetic counselor to review your results. You may choose to do more testing, or not.
If you are a "low risk" patient, with no special concerns or family history of concern, it is most likely your provider can do one of the following tests. If however, you are "high risk" (which just means there is something different about your personal or family history) then it is best to seek genetic counseling first to assure you have complete and correct information about the concern and all of your options.
Screening tests for low risk women are listed below, and definitions are below that:
Serum Integrated Screen
This test involves two blood tests taken at different times of your pregnancy. It can be done in your doctor's clinic. This test is used to estimate the chance of your baby having Down syndrome, trisomy 18 or spina bifida. The first blood draw is about 11 - 13 weeks of pregnancy. The second blood test is during the second trimester of pregnancy, between 15 and 20 weeks of pregnancy. This test can estimate the chance of trisomy 18, detect 90% of cases of Down syndrome, and detect 80% of cases of spina bifida. It does not look for other birth defects.
This test involves a single blood test that can be done in your doctor's clinic. It also estimates the chance for Down syndrome, trisomy 18 or spina bifida. This test is done between 15 and 22 weeks of pregnancy. Like the serum integrated screen, it estimates the chance of trisomy 18, but detects only 84% of cases of Down syndrome, and detects 80% of cases of spina bifida. It does not look for other birth defects.
Cell Free DNA (cfDNA)
This involves just one blood draw that can be done in your doctor's clinic. This test can be done any time after the 12th week of pregnancy and can tell you the risk of your baby having Down syndrome, trisomy 18, trisomy 13, or sex chromosome issues. This test is often offered to women who are of higher risk, such as those who are 35 years or older. This is the best blood test we have for checking an unborn baby's chromosomes. This test does not give information about spina bifida, and an additional blood test will be offered to you during weeks 15 and 22 of your pregnancy for that condition.
Expanded Carrier Screening
This is a blood test that can be done at any time during your pregnancy or your life. This tests to see if you could pass certain rare genetic disorders on to your children. A carrier is someone who has one copy of a gene for a hereditary disease and a normal copy. If you are a carrier, then we need to test your partner. If your partner is also a carrier for the same condition, then your children have a 25% chance of having that disorder.
Below are definitions of common words you may hear or read about in terms of prenatal screening:
Down syndrome - also known as trisomy 21, this is a genetic disorder in which there is a third copy of chromosome 21. Learn more
Trisomy 18 - also known as Edward's syndrome, this is a genetic disorder in which there is a third copy of chromosome 18. Learn more
Spina bifida - a birth defect that occurs when the spine and spinal cord do not form properly. The problems a child with spina bifida will have depend on the location of the opening and if the spinal cord or nerves were affected Learn more
Chromosome - the structures in our cells that carry genetic information. Chromosomes come in pairs; one chromosome comes from the mother and the other copy from the father. Humans typically have 46 chromosomes total, in 23 pairs. Having extra, or missing, chromosomes can have different health effects, depending on which chromosome is affected.
Sex chromosome - the pair of chromosomes that determine whether the baby will be a boy or a girl. Two X chromosomes are seen in a girl, while an X and Y chromosome are found in a boy. However, when there are extra or missing sex chromosomes, babies can have health effects.
Gestational age - describes how far along the pregnancy is in terms of weeks. It is measured from the first day of the woman's last menstrual period to the current date.
Diagnostic test - these are "yes" or "no" tests that can confirm whether or not your baby has a genetic condition or birth defect. These are different from screening tests. Screening tests tell us the chance something is there. None of the tests described above are diagnostic tests. Before you have a diagnostic test, you should meet with genetic counseling to have as much correct information as possible, so you can then decide if you want a diagnostic test or not.