For several years as a prenatal genetic counselor, I spent my days discussing pregnancy risks with expecting women and couples. One of the most common reaons for referral was a screen-positive result on maternal serum screening. (At the time, first-trimester screening was in its infancy, so I'm referring here to second trimester screening) This blood test is performed in the second trimester of pregnancy and outputs the risk for spina bifida, Down syndrome, and trisomy 18 based on three (or, now, four) maternal serum proteins.
It's intention is to flag those pregnancies that are at higher risk, so that further investigation with high-resolution ultrasound and/or amniocentesis can be offered. It's real effect is to scare the living daylights out or pregnant women, who often assume that their baby definitely has the condition instead of simply an increased risk for the condition.
Here is a common risk-communication scenario:
The results of the maternal serum screen indicate a 1 in 100 chance for the fetus to have Down syndrome. That's 1%. Or, turned around, there is a 99/100 chance or 99% chance that the fetus does NOT have Down syndrome.
However, to find out for certain, amniocentesis must be performed. There is a chance of pregnancy loss with amniocentesis that's usually quoted around 1 in 200, or 0.5%.
The risk of pregnancy loss is lower than the risk of Down syndrome. If you were making a decision based solely on the numbers, it makes sense to have the amniocentesis performed.
However, it should not surprise you that people making decisions about their pregnancy and future child aren't concerned only with the numbers. They are concerned about lots of other factors: facing the possibility of a child with disabilities, making a choice about abortion, miscarrying a healthy pregnancy, getting poked with a big needle. I think you get the picture.
Often, it boils down to what I like to call the "rock or a hard place" question. Would you feel worse if you delivered a child with Down syndrome and didn't have the diagnosis prenatally, or if you found out you experienced pregnancy loss because of amniocentesis?
Nobody wants to make that kind of decision, and nobody does it based just on the numbers. Throw in the complexity nature of families, reproduction, marital relationships, past experience with disability, and so many other factors, and you can understand why it's call genetic counseling.
Nicole Teed, MS, CGC