What is 1st Trimester Down Syndrome Screening / Double Marker?
First-trimester screening is a prenatal test that offers early information about a baby's risk of certain chromosomal conditions, specifically, Down syndrome (trisomy 21) and extra sequences of chromosome 18 (trisomy 18).
First-trimester screening, also called the first trimester combined test, has two steps:
- A blood test to measure levels of two pregnancy-specific substances in the mother's blood — pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (HCG)
- An ultrasound exam to measure the size of the clear space in the tissue at the back of the baby's neck (nuchal translucency)
Typically, first-trimester screening is done between weeks 11 and 14 of pregnancy.
Using your age and the results of the blood test and the ultrasound, your health care provider can gauge your risk of carrying a baby with Down syndrome or trisomy 18.
If results show that your risk level is moderate or high, you might choose to follow the first-trimester screening with another test that's more definitive.
The double marker test, also known as maternal serum screening, is part of a more comprehensive screening called the first-trimester screening. It’s not a definitive test. Instead, it’s classified as a predictive test, which means its results report the likelihood of chromosomal abnormalities.
Specifically, this test screens for blood levels of both free beta-human chorionic gonadotrophin (beta-hCG) and pregnancy-associated plasma protein-A (PAPP-A).
In a typical pregnancy, there will be either 22 pairs of XX chromosomes in female fetuses or 22 pairs of XY chromosomes in male fetuses.
A trisomy is a chromosomal condition in which there are extra chromosomes, such as the following:
- Down syndrome. This common trisomy is also referred to as trisomy 21 because there’s an extra copy of chromosome 21.
- Trisomy 18 and trisomy 13. These common chromosomal abnormalities involve an extra copy of chromosome 18 (Edward’s syndrome) or chromosome 13 (Patau’s syndrome).
Levels of hCG and PAPP-A may be either higher or lower than “normal” in pregnancies with these chromosomal abnormalities.
However, blood levels alone don’t produce your results. Instead, the blood test is used along with an ultrasound called a nuchal translucency (NT) scan, which examines the clear tissue at the back of your baby’s neck.
Why a double marker test is done
The first-trimester screening — double marker test and NT scan — isn’t mandatory. That said, the screening (and others like the cell-free DNA test) is recommended if you’re over the age of 35 or may have an elevated risk of chromosomal issues, such as if you have a family history of certain conditions.
It’s important to remember that the result only tells you whether there’s an increased risk of trisomies. It doesn’t definitively determine whether your baby has any abnormalities.
Before deciding whether you want a double marker test, you might ask yourself what the results would mean to you in the long run.
- Would knowing about possible abnormalities ease or worsen your anxiety?
- Would you want to go for more invasive testing if you receive a result that indicates heightened risk?
- Would the results change how you’d manage your pregnancy?
It’s all about personal choice and your own health history, so there’s really no right or wrong answers to your questions.