Maternal serum screen (MSS) is a screening test that your provider can arrange in the second trimester of pregnancy. MSS uses a combination of maternal age and blood work to give information about trisomy 21 (Down syndrome) and trisomy 18.

MSS will not screen your pregnancy for open neural tube defects (eg. spina bifida). A detailed anatomy ultrasound scheduled between 18-20 weeks' gestation will assess the development of the fetal spine.  

How the test works

If you choose to have MSS, a blood test will be done from 14 weeks and 20 weeks 6 days of pregnancy to measure the levels of 4 different chemicals that are present in your blood at this point:

• human chorionic gonadotropin (hCG) – a hormone made by the placenta during pregnancy
• alpha-fetoprotein (AFP) – a protein produced by the baby's liver
• inhibin-A – a protein produced by the baby and the placenta
• unconjugated estriol (uE3) – a form of estrogen that increases during pregnancy

The results of the blood work will be combined with your age at delivery (or the age of your egg donor) to give information about the chance  of trisomy 21 (Down syndrome) or trisomy 18 in pregnancy

It is important to remember that no screening test can give a definite “yes” or “no” answer about these conditions.

The benefits of MSS

• screening tests pose no risk to pregnancy
• can be done if screening in the first trimester was missed. In areas where a nuchal translucency (NT) ultrasound is not available, a first trimester dating ultrasound improves the accuracy of MSS and the management of pregnancy

The limitations of MSS

• screening tests do not give you a “yes” or “no” answer, they will tell you the chances of having a baby with a specific condition
• screening tests are not 100% accurate – false positive and false negative results are possible
• If your pregnancy was conceived through in vitro fertilization (IVF) and the embryos were screened with Preimplantation Genetic Testing for Aneuploidy (PGT-A) during that process, then traditional screening such as MSS is not recommended during your pregnancy¹. PGT-A involves screening embryos for chromosome differences, including trisomy 21 and trisomy 18 and it is a highly effective screen, although it is not diagnostic. 

Screening results

Your MSS result will be shared with your provider within approximately 5 working days. Talk to your provider about how they plan to share these results with you.

When you receive your MSS result it will say that you are a “screen positive” or a “screen negative”. Your provider will review the result with you and make a plan for follow-up if necessary.

Detection rate

The trisomy 21 (Down syndrome) detection rate of MSS is approximately 81%. This means that 81% of patients carrying a baby with trisomy 21 will get a screen positive result using MSS. Because screening cannot detect 100% of cases, approximately 19% of pregnancies with Down syndrome will receive a screen negative result using MSS. More information about screening performance is available here. 

Please note that in response to the COVID-19 pandemic, a temporary change has been made to the MSS cut off to make the trisomy 21 detection rate similar to that eFTS. 

References: 

¹Committee Opinion No 406: prenatal testing after IVF with preimplantation genetic testing for aneuploidy. J Obstet Gynaecol Can 2020