There are several genetic screening tests available: blood tests, ultrasound, cvs, and amniocentesis. These are optional tests and all have advantages and disadvantages.
The blood test goes by different names, the AFP, triple screen, quad marker, etc. These tests will not tell you if the baby has a problem, only if there is an increased risk of problems. There is a high false positive rate. This means that most of the time when the test is abnormal, the baby is fine, and the test is wrong. A positive test is any risk of abnormality greater then 1:190. The test will give you a ratio of risk. If it comes back 1:100, for example, that means that there is a 1% chance the baby has a problem, 1:50 is a 2% risk of a problem. So 1:190 would be a very small risk. This test can be done with a single blood draw at 16 weeks, or it can be split into two tests, one at 12 weeks and one at 16 weeks. Splitting the test gives a slightly smaller false positive rate. The problem with the 12 week test is that the follow up test, amniocentesis cannot be done until 16 weeks. There is another test called CVS that can be done at 12 weeks but it is not available in all areas and carries more risk than the amniocentesis.
Ultrasound is another method that can be used to check for problems with the baby. An ultrasound can be done at 12 weeks by looking at the thickness of the neck; in conjunction with the blood test this can help predict risk. Again if it comes back abnormal you may not be able to do the confirmatory test until 16 weeks unless CVS is available and you choose to do it. Ulrasound in the mid-second trimester can also look for abnormalities. See anatomy screening ultrasound. This is almost universally done.
CVS is chorionic villus sampling. It is done by putting a needle into the placenta at 12-14 weeks and taking a biopsy. The advantage of CVS is that it can be done earlier than amniocentesis. The disadvantages are that it has a higher chance for miscarriage, can give false negative results, and is not available in all areas.
Amniocentesis can be done at 16 weeks or later. It involves putting a needle through the tummy into the uterus and withdrawing fluid from around the baby. Ultrasound is used to guide the needle. It has a 1:200 risk of causing a miscarriage which is a small chance but large enough that it does happen from time to time. The fluid is then sent to a lab where fetal cells can be grown in culture and the baby’s chromosomes can be analyzed. It is very good at finding some genetic abnormalities such as Down’s syndrome, other trisomies, and neural tube defects (problems with the brain stem or spinal cord). The test takes about 10 days to come back after the procedure is done.
None of these techniques will find problems that can be fixed. If you find out your baby has one of these problems, then the decision is whether to carry the pregnancy or have an abortion.
My advice for patients is to consider this carefully. The first question you should ask yourself, is would you have an abortion if you found out your baby has one of these types of problems. If the answer is yes, then you should strongly consider getting the tests. If you would carry the baby regardless of the disability, then the information becomes less useful. You will need to decide how much risk you are willing to take to find out the information prior to delivery. If you are not willing to do the amniocentesis or cvs, then I would recommend against the blood test. Because the blood test will not tell you for sure if you are having a baby with an abnormality, it tends to do more harm than good if it comes back positive and the follow up test is not done.
I will share with you my personal story. I have 3 children. The first pregnancy my wife and I did the blood test because we didn’t even think about it; it came back normal. The second child we again did the blood test without thinking. This time it came back abnormal. There was a 1:34 chance the baby had Down’s syndrome. After much agonizing, my wife and I agreed that we did not want the amniocentesis, we were going to have the baby no matter what so we were not willing to take the risk of losing the baby just to relieve our anxiety. We worried throughout the pregnancy even though we knew that the chance of Down’s syndrome was very small. That baby was born healthy and without Down’s syndrome. The third time my wife became pregnant, we did not do the genetic testing as we learned from our previous experience that it was not helpful to us. That is my personal experience. I share it not because it is the right answer for everyone; it is not. But for some people, it might help to understand the ramifications of the blood test.