Prenatal screening using maternal blood has been in use since the late 1970’s as a way to identify women who are at an increased risk to have a baby with certain conditions. The testing has come a long way since then, and women are now faced with multiple options, and often confusing or scary sounding results.

Maternal serum screening comes in many forms. This testing has evolved over the years from measuring a single protein called AFP during the second trimester, to measuring several proteins at different times during the pregnancy, combined with ultrasound measurements. This testing goes by many names (some now outdated), including X-AFP Screen, Quad Screen, Triple Screen, Serum-Integrated Screen, and Combined Screen. 

How it Works

Though they go by many names, the basis of these tests are the same. Different pregnancy-related proteins increase and decrease in mom’s blood throughout the pregnancy at different times. If the baby has certain conditions, that can affect the levels of proteins in mom’s blood. Some conditions increase or decrease the level of some proteins while not affecting others. By looking at the patterns of proteins at certain times during her pregnancy, the lab can determine the chance that the baby has one of these conditions. This chance can range from as low as 1 in 100,000 (0.001%), to as high as 1 in 3 (33.3%). Most labs have a cut-off number for each condition. Anyone falling below that risk (say, 1 in 200), will be told she has a “negative” or “low risk” result, while anyone above that risk will be told she has a “positive” or “high-risk” result. 

Though the words “positive” and “negative” may be used to describe your results, the key thing to keep in mind is that they are still just increased or decreased chances. If we have 100 ladies in a room, and all of them have a 1 in 100 chance of having a baby with Down syndrome, then 99 of those ladies will NOT have a baby with Down syndrome. In fact, the majority of women who are told they have “positive” results on these tests go on to have perfectly healthy babies. 

What Conditions Can They Detect?

Most of these tests assess the risk for a certain birth defects and genetic conditions. These include neural tube defects (such as spina bifida), and common chromosomal conditions (usually Down syndrome and Trisomy 18). Some tests include a risk assessment for Smith-Lemli-Opitz syndrome, which is an extremely rare genetic condition. Again, most pregnancies that screen “positive” for these conditions will not be affected by them.  

What Happens Next?

If your test indicates you may have a higher risk to have a baby with a certain condition, your doctor may refer you to a Genetic Counselor, or a Perinatologist (a doctor specializing in high-risk pregnancies). There, the results will be explained to you, and you may be offered further testing. There may be other blood tests available, such a NIPT, that may give a more accurate risk assessment (but not a definitive answer). An ultrasound may provide more clues as to whether the baby has the condition in question, but a normal ultrasound does not guarantee a healthy baby. A diagnostic procedure, such as CVS or amniocentesis, may be offered to analyze the baby’s chromosomes or perform other genetic or protein-related tests. 

All of these tests and procedures are completely optional. It is up to you as to how many more tests you want to have done. What you do with that information once you have it is your decision as well. Your genetic counselor or physician can explain the benefits, risks and limitations of each available option so you can make an informed decision about how to proceed.

Reasons for False “Positive” Results

There are many reasons these results may indicate a higher risk for a certain condition when the baby is not affected. These results are based on statistics observed over tens of thousands of women over time, but some women are just above or below average for no apparent reason. Maternal or placental factors can sometimes affect protein levels. A loss of a twin early in the pregnancy can invalidate the testing, and some testing is less-reliable or not possible on pregnancies of multiples (twins, triplets, etc). 

Sometimes an error on the form used to submit the test can be enough to throw-off the results. The mother’s age, weight, due-date and ethnicity are all important factors used in calculating the test results. I have seen results go from extremely high risk to extremely low just by correcting a due date. Your doctor or genetic counselor should always confirm all the factors that were used to calculate the results.

Don’t Jump to Conclusions

Hearing the word “positive” when it comes to any medical test can be panic-inducing, especially during pregnancy. It’s hard not to imagine the worst when presented with uncertain information. It is important to remember that screening tests such as these are only a tool used to help identify which pregnancies may need a closer look. Most women are able to breathe a sigh of relief after that closer look is taken.


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