A sequential screening is a series of tests your doctor may recommend to check for neural tube defects and genetic abnormalities. It involves two blood tests and an ultrasound.
When you’re pregnant, the hormones and proteins your baby produces can go back into your blood. Your body also starts making additional hormones and proteins to support your baby’s growth. If these levels become abnormal, they can indicate possible abnormalities with your baby’s growth.
The sequential screening can test for the possibility of open neural tube defects. Examples of these defects include spina bifida and anencephaly, when the brain and skull doesn’t properly form.
The screening can also detect Down syndrome and trisomy 18, two chromosomal abnormalities.
How does the sequential screening test work?
A sequential screening test involves two parts: blood testing and an ultrasound.
Doctors will take two blood tests for the sequential screening. The first is between weeks 11 and 13 of your pregnancy. The second is usually performed between weeks 15 and 18. But some doctors may perform the test as late as 21 weeks.
Testing a mother’s blood in the first and second trimesters can provide greater accuracy.
An ultrasound machine transmits sound waves that send back wavelengths that allow a machine to produce an image of your baby. A doctor will perform an ultrasound between weeks 11 and 13. The focus is on the fluid-filled space in the back of your baby’s neck. Your doctor is looking for nuchal translucency.
Doctors know that babies with genetic abnormalities like Down syndrome often have a greater buildup of fluid space in their necks during the first trimester. This noninvasive screening isn’t a definitive diagnosis, but measuring nuchal translucency can support other information from a blood test.
Sometimes your baby may not be in a good position for the screening. When this is the case, your doctor will ask you to come back at another time to try the ultrasound again.
What does the sequential screening test?
The first blood test in the sequential screening measures pregnancy-associated plasma protein (PAPP-A). In the first trimester, doctors associate low levels of PAPP-A with greater risk for neural tube defects.
A laboratory worker will analyze the results by using the PAPP-A results, plus the nuchal translucency measurements to determine a woman’s risk.
The second blood test measures for the following.
- Alpha-Fetoprotein (AFP): The baby’s liver mostly secretes this protein that passes over into the mother’s blood. Too-high and too-low levels of AFP have been associated with birth defects.
- Estriol (uE3): Estriol is the highest amount of circulating hormone in a woman’s blood while she’s pregnant. Low levels of this hormone are associated with increased risk for Down syndrome and trisomy 18.
- hCG: Human chorionic gonadotropin (hCG) is also known as the “pregnancy hormone.” The body usually makes less of this hormone in the second trimester than in the first. High levels of hCG are associated with Down syndrome, but there are other reasons levels may be high. Low levels are associated with trisomy 18.
- Inhibin: Doctors don’t know exactly the role this protein plays in pregnancy. But they do know that it increases the reliability of the sequential screening test. High levels are associated with Down syndrome while low levels are associated with trisomy 18.
Every lab uses different numbers to determine highs and lows for these tests. Typically, the results are available within a few days. Your doctor should give you a report that explains the individual results.
How conclusive is the sequential screening test?
The sequential screening test doesn’t always detect every genetic abnormality. The accuracy of the test depends on the screening results, as well as the skill of the doctor performing the ultrasound.
The sequential screening test detects:
- Down syndrome in 9 out of 10 babies tested
- spina bifida in 8 out of 10 babies tested
- trisomy 18 in 8 out of 10 babies tested
The sequential screening results are a way to signal that there could be a genetic abnormality in your baby. Your doctor should recommend other tests to confirm a diagnosis.
An estimated 1 in 100 women will have a positive (abnormal) test result after the first blood test. This is when the proteins measured in the blood are higher than the screening cutoff. The lab that conducts the screening test will issue a report to your doctor.
Your doctor will discuss the results with you and usually recommend more conclusive screenings. An example is amniocentesis, which involves taking a sample of the amniotic fluid. Another is chorionic villus sampling (CVS), which involves taking a small sample of placental tissue.
If the first blood test has proteins that are under the screening cutoff, a woman can have a repeat test in her second trimester. If the protein levels have gone up after the second testing, a doctor will likely recommend genetic counseling. They may recommend further testing, such as an amniocentesis.
Negative test results mean you are at a lower risk to have a baby with a genetic condition. Remember, the risk is lower, but not zero. Your doctor should continue to monitor your baby throughout your regular prenatal visits.
A sequential screening is one of several tests that can help you understand potential genetic abnormalities in your baby. If you’re expecting, some additional steps you could take include:
- Talking to your doctor to ask if you could benefit from a sequential screening.
- Asking your doctor to explain your results and clarifying any additional questions you may have.
- Speaking with a genetic counselor if your test results are positive. A counselor can further explain the likelihood of your baby being born with an abnormality.
If you have a family history of Down syndrome or are at increased risk for having a baby with genetic abnormalities (like being older than age 35), the screening may help provide peace of mind.
Are sequential screening tests standard for all pregnant women, or only for high-risk pregnancies?
Prenatal testing can be offered to all pregnant women. However, it’s more strongly recommended for women that are at higher risk of having a child with birth defects, including women ages 35 and over, those with a family history of birth defects, women with diabetes, and those exposed to high levels of radiation or certain medications.Katie Mena, MDAnswers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.