Very important to know: in prenatal testing there is a difference between diagnostic tests [amniocentesis and chorionic Villus Sampling (CVS)] and screening tests.
SCREENING TESTS
Screening tests are made up of ultrasounds and blood tests that refine the odds of your baby having one or another disorder.They don't give you a definitive diagnosis. And they could be wrong. They are designed to cast a wide net so if there is a suspected problem you can choose to pursue a definitive diagnostic test.
There are two major screening tests -- one performed at the end of the first trimester and another performed at 15 to 20 weeks.
Sometimes offered under the brand name Ultrascreen, the first trimester screening is an ultrasound that measures the fetus's nuchal translucency (NT) (fluid around the back of the fetus's neck), and the blood levels of freeBeta-hCG and PAPP-A.
According to the Ultrascreen site, the blood test and nuchal scan together can detect 91 percent of Down syndrome, 97 percent of Trisomy 18, 40 percent of heart defects and some other birth defects.
The Maternal Serum Alpha Fetoprotein test (AFP) is performed during the second trimester -- between 15 and 20 weeks. It is less accurate than the Ultrascreen (with the exception of neural tube problems). The AFP test is 80 percent accurate for neural tube defects, 60 percent to 80 percent accurate for Down syndrome and 60 percent to 80 percent accurate for Trisomy 18.
This page from Obstetrix offers a decent overview of the screening and diagnostic tests available.
Interestingly, one of our genetic counselors (sad that we have more than one, isn't it?) told us that the first trimester screening, which includes the nuchal fold scan and measuring the levels of a few different hormones in your blood is more accurate than the second trimester screening. That means the odds of it giving a so-called "false positive" are lower, and that it catches more cases of genetic disorders than the second trimester screening.
But the first trimester screening is newer, so it's not recommended as often, which sucks, because it's a better screening test and it catches problems earlier in the pregnancy.
Here's an interesting discussion (although dated: December, 2001) about why first trimester screening has not caught on in the United States as quickly as it has in some other areas of the world.
DIAGNOSTIC TESTS
On the other end of the scale, diagnostic tests include amniocentesis and chorionic villus sampling (CVS). Diagnostic tests give you a yes or no answer, but they are also invasive and there is small chance of pregnancy loss.
Amnio can be performed at the earliest at 16 weeks. It is performed by inserting a needle into the pregnant woman's belly into the womb and the amniotic sac, guided by ultrasound. The needle is used to extract a small amount of amniotic fluid for testing. It is in invasive procedure and the odds of complications are said to be around 1 in 200 to 1 in 300.
However, it offers definitive results. If the amnio test results show that you are all clear, then you are all clear. If they show you have a problem, then you have a problem.
Of the something like 27 cells they tested for our baby in our amnio sample, all of them showed Trisomy 18. (Which meant there was no mosaicism -- where some cells were affected and others were not. Our baby had full Trisomy 18).
Amnio tests for trisomies (21, 18 and 13) and they also test for neural tube defects which can result in conditions such as Spina Bifida.
In retrospect I wish we had done the CVS test for our IVF pregnancy. It is a little more invasive -- it actually samples genetic material from the placenta, using a needle inserted either through the cervix or through the belly like amnio. People often say that this test has a higher complication rate than amnio -- something like a 1 in 100 loss rate. However, in the hands of experienced doctors, the complication rate is about the same as for amnio.
If you are considering this test, I recommend booking it as soon as you get a positive pregnancy test, or at the latest when you see the heartbeat at the six week ultrasound. That's because these appointments fill up fast, and you might miss your window (it is performed between 10 weeks and 13 weeks) if you wait to book the test until after your first OB appointment. You can always cancel the test later if your blood test results are spectacular or if you just change your mind.
When looking for a doctor to do this procedure, ask how many CVS procedures he/she performs each year. I wouldn't go to a doctor who did fewer than 50 a year. And I would prefer a doctor who did over 200 of them a year.
Also, ask what the doctor's loss rate is for this particular procedure. (If you call, ask to speak to one of the genetic counselors and tell her you need to know -- off the record -- the loss rate and which doctor SHE would choose for her procedure. You might even get a genetic counselor who tells you about doctors in another practice.)
There are doctors who are known for their low rates of loss for CVS. Here are some names I have heard: Dr. Williams at Cedars Sinai in Los Angeles. Also Dr. Goldberg or Dr. Otto at California Pacific Medical Center in San Franciso.
Keep in mind that you will need to have a day of bed rest or couch rest after either the amnio or the CVS. So if you are traveling for one of these procedures, you may want to book a hotel. If it were me, I'd stay there for a few days just to be safe. Get some magazines, your TV remote, your laptop, and take to your bed.
For our FET pregnancy, we did an hour plus drive up to San Francisco to see Dr. Goldberg at California Pacific Medical Center. I was looking forward to finally being able to exhale and be happy again. But I should've known by then that that kind of instant relief just isn't in the cards for me when it comes to the whole fertility game.
