For women who want information about the risk of down syndrome but who wish to avoid moving directly to an invasive test such as chorionic villus sampling or amniocentesis, first trimester testing offers an early screening option, although it cannot give a definitive result.

First trimester screening is performed between 11 and 14 weeks of pregnancy and uses a combination of a blood test and an ultrasound to determine the risk of your baby being born with down syndrome or some other chromosomal abnormality. The ultrasound measures the thickness of the skin at the back of your baby’s neck, called the nuchal fold, which is thicker in babies with down syndrome. On the same day as the scan, you will have blood drawn and tested for levels of pregnancy associated plasma protein A (PAPP-A) and human chorionic gonadotropin (hCG). Using the combined results of blood tests, age, and nuchal translucency measurement, the doctor will be able to calculate your baby’s risk of down syndrome. You will receive the results a few days after the test. Your risk will be compared with what the risk of chromosomal abnormality would be for a woman of your age. Your new risk may be higher, lower, or the same as your risk based on your age alone. The test is called positive if your risk of having a baby with down syndrome is higher than a preset cutoff point, usually 1 in 250. A positive test does not mean that your baby has down syndrome, only that the risk is greater. First trimester screening will also detect most babies affected by trisomy18.

Test Accuracy

In women over the age of 35, first trimester screening will detect 85 to 95 percent of babies with down syndrome. However, it can also give a false negative result (suggesting the baby is at low risk down syndrome when in fact the baby is affected). First trimester screening will be negative in between 5 and 15 percent of pregnancies where the baby has down syndrome.

How Age Affects Results

The test is more likely to come back positive as you get older. In women over the age of 35, a quarter of tests will be positive. Although most of these mothers will still have a normal baby, their results may make CVS or amniocentesis a reasonable option. However, 75 percent of women will have a negative test and can feel more comfortable about avoiding CVS or amniocentesis.

Results

If your test is positive

Even with a positive result. the chance of your baby having down syndrome is still very small You will be told the actual statistical risk of your baby having down syndrome; you can compare this risk to the risk of miscarriage with CVS or amniocentesis. Only one of these tests can give you a definitive answer.

If your test is negative

If your first trimester screening test is negative, then your baby has a very, very low chance of having down syndrome. However, if you are still nervous after the screening test, you can have either CVS or amniocentesis. Both of these tests will give you a definitive result.

Nuchal Translucency Scan

• The test is usually performed at 11-14 weeks of pregnancy. The doctor or technician (sonographer) performing the scan will place the ultrasound probe on your belly and look for an area at the back of the baby’s neck called the nuchal translucency. The person doing the scan will mark two points and take a measurement of the thickness at this point. If the measurement is small, the risk of down syndrome is low (the exact measurement depends on the size of the baby). A larger thickness at this point generally indicates that there is a higher than normal risk of the baby having down syndrome.
• Normal nuchal scan shows the outline of the baby, two crosses marking the back of the neck. A narrow gap suggests a low risk of down syndrome. The nuchal translucency scan result must be combined with the results of your blood tests to accurately assess your baby’s risk of down syndrome.
• Abnormal scan shows a thickening at the back of the neck (marked with two crosses). The greater the thickness, the higher the (risk of down syndrome. This test is very dependent on the skill of the sonographer. A poor ultrasound can miss down syndrome or give a positive result with a normal baby.