Pregnancy tests - maternal serum screening

The maternal serum screening (MSS) test is a blood test offered to pregnant women. It helps determine the chance of their unborn child having Down syndrome (trisomy 21), Edward syndrome (trisomy 18) or a neural tube defect.

Maternal serum screening may be done in the first trimester of pregnancy, when it is combined with an ultrasound; this is called the combined first trimester screening test. It may instead be done in the second trimester, which is called the second trimester maternal serum screening test.

What MSS screens for#

MSS can identify pregnancies that are at increased chance of the following conditions:

• Down syndrome (trisomy 21), a condition that results in intellectual disability of varying degrees. It may also cause some physical problems such as heart defects or difficulties with sight or hearing. It is known as trisomy 21 because the baby has three copies of chromosome 21 instead of the usual two.
• Edward syndrome (trisomy 18), a condition that results in physical problems such as growth deficiency, heart defects, digestive system defects and intellectual disability. Babies born with Edward syndrome have a poor prognosis, and many do not live beyond the first few weeks of life. It is known as trisomy 18 because the baby has three copies of chromosome 18 instead of the usual two.
• Neural tube defects, the most common being anencephaly and spina bifida. With anencephaly, the brain does not develop properly and the baby does not survive. Babies with spina bifida have an opening in the bones of the spine that can damage the nerves controlling the lower part of the body, causing weakness and paralysis of the legs and sometimes an inability to control the bowel and bladder.

Neural tube defects are only screened for in the second trimester MSS test, and they can also be diagnosed by routine ultrasound during pregnancy.

This test is only for these conditions#

A low-chance result does not mean the baby is free of other birth defects.

Combined first trimester screening test#

The combined first trimester screening test can identify a pregnancy with an increased chance of Down syndrome (trisomy 21) and Edward syndrome (trisomy 18). It involves a blood test between 9 and 13 weeks gestation and an ultrasound between 11 and 13 weeks gestation.

The blood test measures the amount of two proteins that occur naturally in the mother’s blood during pregnancy. Using ultrasound, a measurement is taken of the amount of fluid in the skin at the back of the baby’s neck.

This measurement is called the nuchal translucency#

All babies at this stage of pregnancy have fluid at the back of the neck, but babies with Down syndrome or Edward syndrome tend to have more. During the ultrasound, the specialist performing the scan may also report whether the nasal bone is present. If the nasal bone is absent, there may be an increased chance of Down syndrome.

Your doctor or medical specialist can order this test. There may be out-of-pocket costs depending on the service you attend, so discuss this with your health professional.

Second trimester maternal serum screening test#

The second trimester maternal serum screening test can identify a pregnancy with an increased chance of Down syndrome, Edward syndrome or neural tube defects. It involves a blood test between 14 and 20 weeks plus 6 days gestation.

However, it is best if the blood is collected between 15 and 17 weeks gestation#

The test measures the amount of four proteins in the mother’s blood. The results are combined with the chance based on the woman’s age to give a final chance of Down syndrome, Edward syndrome or neural tube defects. The result is reported as either an increased chance or a low-chance result.

Understanding your results#

Screening for Down syndrome and other chromosome conditions should be offered to all pregnant women, but the decision to have testing is a personal one. All women have some chance of having a baby with Down syndrome or Edward syndrome, although the chance increases with the woman’s age.

A low-chance maternal serum screening result does not mean there is no chance of a genetic condition in the unborn baby.

It means that the chance is low#

Similarly, an increased chance result does not necessarily mean the baby has a genetic condition. Most women with an increased chance result will not have a baby with Down syndrome, Edward syndrome or a neural tube defect.

Maternal serum screening tests do not diagnose health conditions; they identify women who should be offered further testing to detect whether their baby has a genetic condition. Women with an increased chance result should be offered a diagnostic test, although it is their choice whether to have one.

Diagnostic testing#

Chorionic villus sampling (CVS) and amniocentesis are diagnostic tests that detect chromosome conditions such as Down syndrome and Edward syndrome in the unborn baby.

In most cases an increased chance result does not mean the baby has the condition, only that they are at increased risk. This is why further diagnostic testing is encouraged, as it provides more information about the likely health outcomes for the baby.

Genetic counselling#

Genetic counsellors are health professionals qualified in both counselling and genetics. As well as providing emotional support, they can help you understand a condition and its causes, how it is inherited, and what a diagnosis means for your child’s health and development. They are trained to provide information and support that is sensitive to your family circumstances, culture and beliefs.

Support networks can also connect you with a wide range of support groups and with other individuals and families affected by the same genetic condition. If your child has been diagnosed with a genetic condition, it can be helpful to speak to a genetic counsellor.

Non-invasive prenatal testing (NIPT)#

Non-invasive prenatal testing (NIPT) is an alternative to maternal serum screening. It is a maternal blood test that looks at the baby’s genetic material (DNA), which can be found in the mother’s bloodstream. It can provide information about the risk of conditions such as Down syndrome and some other chromosome problems, and it can be done any time from 10 weeks into the pregnancy.

NIPT is more accurate than combined first-trimester screening or second-trimester maternal serum screening, but it is more costly than those screening tests.

Where to get help#