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  Back Blood-tests Explained


First Trimester Screening


At a Glance

Why Get Tested?

For pregnant women, to assess the risk of your baby having a chromosome disorder, such as Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18)

When to Get Tested?

Usually between 11 and 14 weeks of pregnancy

Sample Required?

A blood sample drawn from a vein in your arm or a blood spot collected on special paper from a finger stick; a nuchal translucency requires a special ultrasound exam.

Test Preparation Needed?

You may be instructed to have a full bladder when having the nuchal translucency ultrasound performed.

The Test Sample

What is being tested?

The first trimester screening is a combination of two blood tests and a special ultrasound that are used to assess a pregnant woman's risk of carrying a baby with Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18). Performing and evaluating them together, plus considering the woman's age, increases both the sensitivity and specificity of the screening results.

  • Pregnancy-associated plasma protein A (PAPP-A) is a protein produced first by the growing placenta. During a normal pregnancy, levels of this protein increase in the pregnant woman's blood until delivery.
  • Human chorionic gonadotropin (hCG) is a hormone produced during pregnancy in large quantities by the placenta. Either free beta subunit or total hCG can be used in first trimester screening. Levels of both usually rise rapidly in the pregnant woman's blood for the first 8 to 10 weeks, then decrease and stabilize at a lower level for the remainder of the pregnancy.
  • Nuchal translucency is measurement made by ultrasound. The ultrasonagrapher measures the fluid collection between the spine and the skin at the nape of the fetus's neck. It is a procedure that requires a specially trained radiologist, proper alignment of the fetus, and careful measurement. It is not a routine ultrasound, and it is not a procedure that is available at every hospital or health facility.

If the results of first trimester screening are cause for concern, diagnostic tests such as amniocentesis or chorionic villus sampling (CVS) may be recommended.

How is the sample collected for testing?

Blood is drawn from a vein in the woman's arm or collected from a finger stick. The nuchal translucency ultrasound may be performed from outside the abdomen (transabdominally) or the probe may be inserted into the vagina (transvaginally).

Is any test preparation needed to ensure the quality of the sample?

You may be instructed to have a full bladder when having the nuchal translucency ultrasound performed.

The Test

How is it used?

The first trimester screening is a combination of tests for PAPP-A, hCG and nuchal translucency that are used to assess the risk that the fetus a pregnant woman is carrying has a chromosome disorder such as Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18). The first trimester screen is one of the options you may be offered for prenatal Down syndrome screening. Other options include the second trimester maternal serum screen and the cell-free fetal DNA test.

One advantage of the first trimester screen is that it gives expecting women the option of earlier screening. However, the first trimester screen does not assess risk of neural tube defects such as spina bifida. This may be assessed in the second trimester as part of the maternal serum screen (triple or quad screen) or on its own.

The screening approach a woman may choose depends on what technology is available and when she first seeks prenatal care. For more information about how first trimester screening fits in with other screening options, see Common Questions #3 and #4.

When is it ordered?

Your healthcare practitioner may offer you the test between 11 and 14 weeks of pregnancy.

What does the test result mean?

A genetic counselor or clinician who can explain the meaning of the results and offer choices about follow up should interpret the screening results.

A mathematical calculation using the results obtained from the PAPP-A, hCG, and nuchal translucency ultrasound is used to determine a numeric risk of a chromosomal defect in the fetus. This risk is compared with an established cut-off. If the risk is higher than the cut-off value (e.g., probability of 1 in 230 or higher), then it is considered a positive screen and the woman may be at increased risk for having a baby with a chromosomal abnormality.

It is important to remember that positive screening tests are not diagnostic of fetal abnormalities. While they do indicate an increased risk, only a very small number of women with a positive first trimester screen have babies who actually have a chromosome disorder.

In pregnancies where the fetus is carrying a chromosomal defect, such as the extra chromosome material that results in Down syndrome or Edwards syndrome, the levels of PAPP-A tend to be decreased, the levels of hCG are significantly increased, and the space at the fetus's neck is larger than normal.

While the first trimester screen can correctly identify approximately 85% of women carrying a fetus with Down syndrome and up to 75% of those with Edwards syndrome, about 5% to 10% of normal pregnancies will have a false-positive result.

If a screening test is positive, more definitive tests are needed to determine and confirm a diagnosis. These may include a diagnostic test such as chorionic villus sampling (CVS) in the first trimester or amniocentesis in the second trimester. While these two procedures are more accurate than screening tests, they are also invasive and carry a small risk of miscarriage and a rare risk of injury to the fetus.

Screening will not detect all cases of fetal abnormalities.

Is there anything else I should know?

Test results are very dependent on nuchal translucency techniques and the accurate determination of the gestational age of the fetus. If the gestational age of the fetus has not been accurately determined, the results may be either falsely high or low.

In multiple gestation pregnancies (twins, triplets, etc.), calculation of the risk of Down syndrome or Edwards syndrome can be difficult because the amount of PAPP-A and free beta hCG is increased. The nuchal translucency ultrasound, however, is an assessment that is unique to each fetus and can be done independently with ultrasound. A woman with a multiple gestation pregnancy should consult her healthcare practitioner about her options.



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