Traditional Screening

Traditional Screening

Traditional screening tests measure protein and hormone levels in your blood and estimate the chance that the developing baby has a chromosome condition.

Because it is a blood test, there is no risk to your pregnancy.

The blood test is often combined with an ultrasound measurement, called the nuchal translucency (NT), to improve result accuracy. This type of testing is generally restricted to certain periods of time during the pregnancy.1

The detection rate is the chance that the test will correctly identify a pregnancy that has one of these conditions. Detection rates with traditional screening may range from 69-96% for trisomy 21, depending on the method used.1

Sometimes a test can indicate a high chance for a chromosome condition when the developing baby does not have it. This is called a false positive result. With traditional screening, about 1 in 20 women might receive a false positive result.1

Traditional Screening
  1. ACOG Practice Bulletin No. 77, January 2007. Obstet Gynecol; 109(1):217-27.

Screening Reminders

No screening test is designed to detect all possible conditions. Screening tests help identify pregnancies with an increased chance of certain chromosome conditions. With screening tests, false positive and false negative results will occur.

Women who have an increased chance on their screening test should consult a healthcare professional who can recommend additional invasive confirmatory or other testing.

Invasive Confirmatory Testing

Invasive confirmatory testing involves a procedure to obtain cells from the pregnancy for testing.1

Invasive confirmatory testing can give a definitive answer about most chromosome conditions in a pregnancy.

It is most often performed for pregnancies at increased chance for a chromosome condition.

There are two types of invasive procedures, chorionic villus sampling and amniocentesis.

Chorionic Villus Sampling (CVS)2

CVS can be done between 10-13 weeks of pregnancy.

This procedure removes a small sample of cells from the placenta using a needle (through the abdomen) or narrow tube (through the vagina).

All chromosomes within these cells are studied to see if there are any extra or missing.

CVS can provide greater than 99% detection rate for all chromosome conditions.

There is a small chance for miscarriage following the procedure.


An amniocentesis is performed after 15 weeks of pregnancy. Amniotic fluid contains cells from the developing baby, and a small sample of this fluid is removed using a needle that is inserted through the belly. All chromosomes within these cells are studied to see if there are any extra or missing.

Amniocentesis can provide a 99.8% detection rate for chromosome conditions.

This procedure also carries a small chance for miscarriage.

Why might I consider CVS or amniocentesis

According to RCOG (Royal College of Obstetrics and Gynaecologists)4, you might consider CVS or amniocentesis if you:

  • have had a high chance screening test for Down’s syndrome
  • want to know for certain whether your baby has a genetic disorder or not
  • have had a previous pregnancy affected with a genetic disorder
  • or your partner have one or more relatives affected with a genetic disorder, which means you are at greater risk of having a child with a genetic disorder, such as cystic fibrosis, thalassaemia or sickle cell disease
  • have received a result from a scan which shows certain abnormal ultrasound features, such as fluid collection at the back of the baby’s neck (nuchal translucency), or a congenital heart defect which indicates the baby may have a disorder such as Down syndrome. This may be suspected on a scan at 18–22 weeks
  • want to know for certain whether your baby has a genetic disorder or not

Some women may choose invasive confirmatory testing even if there is not a high chance of a chromosome condition in their pregnancy.

  1. ACOG website: (082516)
  2. Chorionic villus sampling. Retrieved from
  3. Amniocentesis. Retrieved from