Integrated prenatal screening combines your age, measurements from the nuchal translucency ultrasound and two blood tests to estimate the chance of having a baby with Down syndrome, Trisomy 13 or Trisomy 18. It then gives a numerical estimate of the chance of having a baby with one of these conditions, such as 1/2000 or 1/8.

In addition, unlike the first trimester screening, integrated prenatal screening will determine if there is an increased chance of having a baby with an open neural tube defect.

Integrated prenatal screening requires two separate blood tests. The first measures the level of a protein called pregnancy-associated plasma protein-A (PAPP-A) in the mother’s blood. The second blood test measures three proteins: Alpha feto-protein (AFP), unconjugated estriol (uE3), and free beta human chorionic gonadotrophin (β-hCG). In pregnancies with Down syndrome, PAPP-A, AFP and UE3 are often lower than normal and free β-hCG is higher. These results, combined with your age, as well as the nuchal translucency, measured on ultrasound, are used to estimate your chance of having a baby with Down syndrome.

The ultrasound and first blood test are done at Mount Sinai between 11 weeks, 2 days and 13 weeks, 3 days of pregnancy. The second blood test is done between 15 – 21 weeks.

The results of the IPS are sent to your healthcare provider (obstetrician, family doctor or midwife) about 1 week after the second blood test. Results are categorized as either screen positive or screen negative.

The chance a woman will screen positive also increases with her age, because the rate of Down syndrome pregnancies increases as a woman ages. If the test indicates that the chance of having a baby with Down syndrome is greater than 1/200, 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 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 known as a false positive result. Only a small proportion will be affected with Down syndrome. This is known as a true positive result.

Close to 90 per cent of the pregnancies with Down syndrome will screen positive with this test. A less than 1/200 chance of having a baby with Down syndrome is a screen negative result. More than 99 per cent of pregnancies WITHOUT Down syndrome will screen negative.

However, about 10 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

If your prenatal screening test is positive you will have to decide whether to have a diagnostic test to determine if your 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, an amniocentesis can be done between 15.5 – 22 weeks gestation.

Results from IPS offer a lower false positive rate and higher detection rate than FTS, meaning fewer women will be offered an unnecessary amniocentesis.

Integrated prenatal screening also includes a screen for open neural tube defects as part of the second blood test.

The main disadvantage is that screening results from an integrated prenatal screening are not available until the second trimester of pregnancy, which delays decision making about further testing (such as amniocentesis), and thus diagnosis.