First trimester screening combines information from a nuchal translucency measurement (an ultrasound measurement), biochemical analysis of free beta hCG and PAPP-A, and maternal age to calculate a risk for Down syndrome and trisomy 18.
What Is "Screening"?
Most tests are done to identify the presence or absence of a condition; these are called diagnostic tests. Other tests, called screening tests, are not intended to diagnose a condition, but are used instead to identify individuals at an increased risk for a specific condition. Screening tests are often used during pregnancy to estimate the risk of certain birth defects. If the risk is high, a woman can then choose to have diagnostic testing. Screening tests typically do not pose a risk to the pregnancy, while diagnostic testing, through amniocentesis or CVS, carries a risk for complications, including miscarriage.
How Do You Screen for Down Syndrome?
In the past, the risk of having a child with Down syndrome was based only on the age of the mother - the older the mother, the higher the chance of having a child with Down syndrome. For example, a woman at the age of 25 has a risk of about 1 in 1250, but a woman at the age of 40 has a higher risk - about 1 in 94.
It has been found that the levels of certain chemicals in the mother's blood can be used along with the mother's age to better predict the risk that a child will have Down syndrome. This has led to a blood test at 15-22 weeks of pregnancy called maternal serum screening (also known as the triple or quad screen). This screening has been used in clinical practice for more than 20 years. More recently, other chemicals have been used at 11-13 weeks of pregnancy to evaluate the risk of Down syndrome earlier in pregnancy. A specific ultrasound measurement can also be used to evaluate the risk of Down syndrome. In early pregnancy, babies with Down syndrome will often have an increased space at the back of the neck. The ultrasound measurement of this area is called the nuchal translucency.
Who Performs the Ultrasound?
Accurate measurement of the space behind the baby's neck is not a standard ultrasound measurement currently done in doctor's offices. This risk prediction is very dependent on precise measurement of this space, and the most accurate results are obtained when the ultrasound in performed by individuals trained in this specific measurement.
Who Can Have First Trimester Screening?
Most pregnant women between 11 and 13 weeks gestation.
How Is First Trimester Screening Performed?
- A small amount of blood is obtained by either by a blood draw or a finger prick to measure the levels of PAPP-A and free beta-hCG.
- An ultrasound evaluation to date the pregnancy and measure the nuchal translucency is performed.
- The mother's blood sample and ultrasound data is sent to the laboratory for analysis.
- After the laboratory analysis, the results calculate the risk for Down syndrome and trisomy 18.
What Do the Results of the First Trimester Screen Mean?
It is important to remember that this is a screening test and does not provide a diagnosis; instead it calculates the chance the baby could have a problem. A result of "screen negative" or "low risk" indicates that the measurements of nuchal translucency, PAPP-A and free beta-hCG were within normal range and that the chance of the baby having Down syndrome or trisomy 18 is low. A result of "screen positive" or "high risk" indicates that the measurements were not within normal range and that the chance of the baby having Down syndrome or trisomy 18 is increased.
If the Results Show an Increased Risk, What Additional Testing Will Be Needed?
If the first trimester screen indicates an increased risk for either Down syndrome or trisomy 18, you will be offered diagnostic testing to determine if the baby has one of these conditions. The following options are available based on your gestational age:
- 10-12 weeks: Diagnostic testing by chorionic villus sampling (CVS) (PDF)
- 15 weeks or later: Diagnostic testing by amniocentesis (PDF)
How Accurate Is First Trimester Screening?
Current available data indicates that combining the nuchal translucency measurement with analysis of free beta-hCG and PAPP-A along with maternal age can identify 90 percent of pregnancies with Down syndrome.
Is Second Trimester Multiple Marker Screening Necessary After First Trimester Screening?
First trimester screening cannot predict the presence of spina bifida and other neural tube defects. Second trimester alpha-fetoprotein (AFP) screening and/or an ultrasound are necessary to evaluate the risk of the baby having this condition.