Maternal Serum Screening is a single blood test taken between 15-21 weeks gestation to screen for Down syndrome, Trisomy 18, Trisomy 13 and open neural tube defects. Maternal serum screening is for women who have not had first trimester screening or integrated prenatal screening. Women who have had first trimester screening should also be offered part of this test to screen for open neural tube defects only.

In this screen, the biochemical markers measured in the mother’s blood are: alpha feto-protein (AFP), unconjugated estriol (uE3), free beta human chorionic gonadotrophin (β-hCG) and inhibin A (DIA). In Down syndrome pregnancies, the levels of these markers may be different. These results and your age are used to estimate your chances of having a baby with Down syndrome, Trisomy 18, Trisomy 13 and open neural tube defects.

Maternal Serum Screening is arranged by your health care provider and requires a blood test between 15 to 21 weeks of pregnancy.

Results are available within about one week after the blood test, and are given to your health-care provider.

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 the chance for Down syndrome is greater than 1/385, the result is called screen positive. The test will also be called a screen positive 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, but the majority of these women will NOT be carrying a pregnancy 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 77 per cent of the pregnancies affected with Down syndrome will screen positive. A less than 1/385 chance of Down syndrome is a screen negative result, and more than 95 per cent of pregnancies WITHOUT Down syndrome will screen negative.

However, about 5.2 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, Trisomy 13 or open neural tube defects will be detected by the test, meaning results are not absolutely guaranteed.

Approximately 75 per cent of pregnancies with Down syndrome and 60 per cent of pregnancies with Trisomy 18 and Trisomy 13 are identified as screen positive with this blood test.

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, an amniocentesis is usually done after 16 weeks gestation.

This test has the disadvantage of a lower detection rate than first trimester screening and integrated prenatal screening. It is usually offered if prenatal care begins after 14 weeks of pregnancy and misses the opportunity of an early ultrasound to measure the nuchal translucency.

As well, results from MSS are not available until the second trimester of pregnancy, which delays decision making about further testing (such as amniocentesis), and thus diagnosis.

Maternal serum screening also screens for open neural tube defects.