The first trimester screening test combines your age with measurements from a nuchal translucency ultrasound and a blood test. The blood test is typically done on the same day as the ultrasound. The first trimester screening provides an estimate of your chance of a having baby having Down syndrome, Trisomy 13 or Trisomy 18.

First trimester screening takes into account the measurement of the nuchal translucency ultrasound and the mother’s blood test. It then adjusts for the mother’s age and gives a numerical estimate of the chance of having a baby with Down syndrome, such as 1/2000 or 1/8.

First trimester screening does not tell you the chance of your baby having an open neural tube defect. A second blood test in the 2nd trimester would be necessary to determine if you have an increased chance of having a baby with this condition.

First trimester screening measures the levels of two proteins in the mother’s blood: Pregnancy-associated plasma protein-A (PAPP-A) and free beta human chorionic gonadotropin (β-hCG). In pregnancies with Down syndrome, PAPP-A is often lower than normal, and free β-hCG is higher. These results are combined with your age and nuchal translucency ultrasound measurement to estimate your chances of having a baby with Down syndrome, Trisomy 13 or Trisomy 18.

The ultrasound and blood test are done between 11 weeks + 2 days and 13 weeks + 3 days of pregnancy.

The results of the FTS are sent to your healthcare provider (obstetrician, family doctor or midwife) in 7 – 10 days. Results are categorized as either screen positive or screen negative.

The chance a woman will screen positive also increases with her age, because the chance of having a baby with Down syndrome increases as a woman ages. If the test indicates that the chance of having a baby with Down syndrome is greater than 1/350, the result is called screen positive. The test will also indicate a screen positive result if the chance of having a baby with Trisomy 18 or Trisomy 13 is greater than 1/100.

Approximately 4 – 5 per cent of women taking this test will screen positive. The majority of women who screen positive will NOT be carrying a baby affected with Down syndrome. This is also known as a false positive. Only a small proportion of these pregnancies will be affected with Down syndrome. This is know as a true positive.

A less than 1/350 chance of Down syndrome is a screen negative result. More than 99 per cent of pregnancies WITHOUT Down syndrome will screen negative. However, between 10-15 per cent of pregnancies with Down syndrome will also screen negative. This is known as a false negative. Not all pregnancies with Down syndrome, Trisomy 18 or Trisomy 13 will be detected by the test, meaning results are not absolutely guaranteed.

First trimester screening has close to an 85 – 90 percent detection rate for Down syndrome, meaning 85-90 per cent of the pregnancies affected with Down syndrome will screen positive.

If your prenatal screening test is positive you will have to decide whether to have a diagnostic test to determine if the baby truly has Down syndrome, Trisomy 18, Trisomy 13 or another chromosome abnormality. To find out for sure if your baby has one of these conditions, chorionic villus sampling (CVS) is usually done at our centre between 11 & 14 weeks or amniocentesis after 16 weeks.

The main advantage of First trimester screening is early detection, with results of both the ultrasound and screening forwarded directly to your health care provider within 7-10 days. This allows for earlier diagnostic testing and decision-making.

A disadvantage of first trimester screening is that it has a mildly lower detection rate than integrated prenatal screening. 10-15 per cent of pregnancies with Down syndrome screen negative with first trimester screening, compared to 10 per cent with integrated prenatal screening.

The false positive rate is also higher with first trimester screening at 4-5 per cent, compared to 2-3 per cent with integrated prenatal screening.

In addition, first trimester screening does not screen for open neural tube defects. A second blood test is required in the 2nd trimester to determine the chance of having a baby with an open neural tube defect.