What Does First Trimester Screening Test For?

The first trimester screening program at The San Diego Perinatal Center is designed to  screen for Down Syndrome and Trisomy 18 in early pregnancy. The Center also uses high resolution ultrasound to screen for major structural abnormalities of the fetus.

What is Down Syndrome?

Down syndrome is a genetic disorder that occurs when an individual has an extra copy of chromosome number 21. Individuals with Down syndrome have a characteristic appearance, various birth defects and are all mentally retarded to varying degrees.
Down syndrome affects people of all ages and races. A woman’s chance of having a child with Down syndrome increases as she gets older. It is the most common chromosomal abnormality in newborns, occurring in approximately 1 in every 800 births.

What is Trisomy 18?

Trisomy 18, also known as Edward’s Syndrome, is a severe genetic disorder that occurs when an individual has an extra copy of chromosome number 18. Babies with this disorder have multiple severe birth defects and profound mental retardation. Most die within the first year of life though an occasional baby survives into childhood. It affects individuals of all ages and races. A woman’s chance of having a child with Trisomy 18 increases as she gets older. It occurs in approximately 1 of every 3000 births.

How is First Trimester Screening Performed?

First trimester screening is performed between the 11 and 14 weeks of pregnancy. An ultrasound is always performed as part of first trimester screening. The fetus is evaluated for a normal size, heart rate, and some major birth defects that can be detected in the first trimester. In addition, the nuchal translucency is measured. The nuchal translucency is a fluid filled area that is found at the back of the fetal neck. The nuchal translucency measurement can then be used along with the mother’s age to calculate a risk for Down Syndrome and Trisomy 18. An above average nuchal translucency measurement will result in a higher risk calculation. Furthermore, large nuchal translucency measurements have also been associated with birth defects and other genetic syndromes.

In addition to an ultrasound exam and nuchal translucency measurement, patients evaluated at The San Diego Perinatal Center are given the option of blood testing for two chemicals:  free Beta-human chorionic gonadotropin (free Beta-hCG) and pregnancy associated plasma protein-A (PAPP-A). These chemicals are found in the blood of all pregnant women and levels can be used with the nuchal translucency measurement and a patient’s age to calculate a risk for Down Syndrome and Trisomy 18.

What Happens if I Have a Positive Test?

Patients with a positive screening test result have a calculated risk that is above the set cut-off value for either Trisomy 21 or Trisomy 18. The cut-off risk for Down Syndrome has traditionally been set to the risk of a 35 year old woman at the specific gestational age. The cut-off risk for Trisomy 18 has been set at 1:150. It is important to remember that a positive screening test does not mean that your baby has one of these disorders. Patients with a positive screening test will be asked to see a genetic counselor that will explain the results and further options. These options will include invasive diagnostic tests such as chorionic villus sampling (CVS) and amniocentesis.

What Does a Negative Test Mean?

Patients with a negative screening test result have a calculated risk that is below the set cut-off value for either Trisomy 21 or Trisomy 18. It simply means that the risk is low and does not warrant invasive diagnostic testing. It is important to remember that a negative test result does not guarantee that the baby will not have Down Syndrome, Trisomy 18, or any other abnormality.

How good is the testing?

Depending on the study, combined nuchal translucency and serum screening has been shown to detect 75% - 90% of Down Syndrome fetuses. It is less effective in detecting Trisomy 18.

What About Expanded AFP Testing?

The California Expanded AFP Testing (XAFP) program is a blood test that doctors are required to offer all pregnant women in California during the second trimester at between 15 and 20 weeks of pregnancy. Like First Trimester Screening, the XAFP test screens for Down Syndrome and Trisomy 18. However, unlike First Trimester Screening, the XAFP test also screens for birth defects such as spina bifida and abdominal wall defects. XAFP testing is currently considered the standard of care for serum screening of Down Syndrome, Trisomy 18, and fetal anomalies in the state of California.

Even if a patient chooses to have first trimester screening performed, she will still be offered XAFP testing by her doctor at the appropriate time. If a patient chooses to have both first and second trimester screening performed, it is currently not known how best to combine the information from the two tests. Recent studies have suggested that combined testing may result in a higher detection rate but at a higher false positive rate.

You are encouraged to talk to your doctor about our program. If you decide to have First Trimester Screening done, an appointment can be arranged through your doctor’s office. You can also call our office at 858-939-6860 to schedule an appointment directly.

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