When a woman becomes pregnant, she is faced with making decisions about testing for fetal abnormalities. I have frequently seen women make these decisions without much thought, and end up in a quandary. (I have seen some go overboard thinking about whether they should do testing or not and drive themselves crazy with this, but not as often). The tests I am referring to are the screening tests for Down Syndrome, neural tube defects, and trisomy 18.
When I was working in an outpatient obstetrics office, it was my job to discuss the tests with the woman and give her information about the tests. Many times she would say, "I'll do whatever test is available to me" when asked if she wants it done. I really tried hard to discuss reasons why and why not to do the tests to present the full picture, but I felt that many were not listening to what I was saying.
So what tests am I talking about specifically? I am talking about screening tests done in the first and second trimester. Notice I emphasized screening, because none of these tests can diagnose a fetus with Down Syndrome, neural tube defects, or trisomy 18. They can only tell if the fetus may be at higher risk for one of the disorders.
The most common screening test is the Quad Screen. This is a blood test that is done between 14-20 weeks, but preferably between the 15-18th week. The Quad screen looks for fetuses that may be high risk for Down Syndrome, neural tube defects, and trisomy 18. It looks at 4 things in the woman's blood:
- HCG (human chorionic gonadotropin)- this is a hormone produced by the placenta.
- AFP (alpha-fetoprotein)- a protein released by the fetal liver.
- Estriol- the estrogen of pregnancy produced by the placenta and fetal liver.
- Inhibin-A- hormone produced by the placenta.
If one or a combination of these substances are abnormal (either too high or too low) may indicate a fetus at risk for one of the conditions. But, it does not detect all fetuses with these disorders, and sometimes falsely reports a fetus at risk who actually is not. This typically happens in cases with woman with a "positive" Quad screen. In other words, the sensitivity isn't 100%. Actually, the sensitivity is around 75-80%. About 2% of "negative" screens are falsely negative; or the test says the fetus is low risk but in fact does have one of the disorders. The results are reported in a ratio. For instance, it will say the Down Syndrome risk is 1:575 (which would be considered low or negative) or 1:15 (which would be considered high or positive). It will also give separate ratios for the neural tube defects and trisomy 18. This ratio is compared to the woman's age related risk. So for a 35 year old the age risk is about 1:240, but if she gets a ratio result that is lower than her age it is considered low risk.
Believe it or not, but that ultrasound you have around 18 weeks isn't just to determine the sex of your baby. It is also a screening test. The fetal survey (the second trimester ultrasound) looks at all of the fetus's organs and body parts. There are certain markers that are looked for that are associated with Down Syndrome or neural tube defects. I could go on in a separate post just about the fetal survey. I've found that not all ultrasounds are created equal; I've seen some pretty rudimentary ultrasounds and I've seen some very thorough scans. One marker seen are Echogenic foci that are bright spots seen on an organ (typically the heart) that may be associated with Down Syndrome. Sometimes the ultrasound is so sensitive that it's finding echogenic focus on organs that can't be explained, or little is known about.
A newer test, that will likely will become the standard, can be done in the first trimester that screens for Down Syndrome. Different places call this test different things, but I've heard it called Early Risk Assessment, Nuchal Translucency Screening, and First Trimester Screening. Many places only offer this to women 35 and older, or those who are high risk for having a baby with Down Syndrome. This test is done by obtaining a blood sample with an ultrasound between 10 and 14 weeks. The ultrasound measures the nuchal fold on the back of the fetal neck. The blood sample measures:
- PAPP-A (pregnancy associated plasma protein-A)
Again, the results are given in a ratio and interpreted the same way as the Quad screen. One downfall to this screening test is that it can not predict neural tube defect risk, so if a woman wants a screening test for this she has to have the part of the Quad screen that looks at neural tube defects in her second trimester.
So what happens when a woman has a positive screening test? First she'll get an explanation of the results, and offered to speak with a genetic counselor. She can also chose to have a diagnostic test such as the chorionic villus sampling (CVS) or amniocentesis.
And why do I say that women do these tests that are offered without much thought? Because when the test comes back "positive" she has no clue what she wants to do. I feel that a woman should have a good idea what she would do with the information these tests are giving her. I've seen some women totally panic and freeze with fear that something is wrong, and can't come to a decision as to what she should do.
So here are some questions to consider when faced with whether or not to do these tests:
- Would having a child with Down Syndrome, spina bifida, or trisomy 18 change the outcome of your pregnancy?
- Would you feel that you could not adequately care for a child with those disorders?
- Would you terminate a pregnancy knowing you were carrying an affected child?
- Would you want to know if your child had one of these disorders so you could prepare for their care (ie, speak with specialists before the birth, align home help, decide whether to continue working)?
- Is not knowing in advance okay or would it create more anxiety?
I'm curious to hear how screening tests were discussed with you. I've seen some women just told that there is a test that is done and that they will be having it at that visit (so much for informed consent!). I've seen some providers tell women that they shouldn't have it done. Hopefully most are given accurate information that allows a woman to make an informed choice.