This is a question I get asked all of the time. The procedure itself is actually very simple. Using ultrasound as a guide to the fetal position, a small needle is inserted directly into the abdomen, into the amniotic sac and fluid is drawn off. This fluid is composed of fetal urine, lung fluids and lots of fetal cells. This cells are isolated, and that is how the fetus’s genetic code is accessed.
I have a spiel that I use prior to every amniocentesis that I perform. I confirm the indication for the amniocentesis and make sure that the parents have been given other options (including the new non-invasive prenatal testing, although it is not covered by the provincial health insurance). I make sure that they will take it easy for the next twenty-four hours and who to contact if they have any concerns, including leaking, bleeding or contractions. Finally, I quote that there is a 1 in 200 risk of pregnancy loss associated with amniocentesis. That is the BC standard answer, and probably standard in many centres.
However, of some patients, the standard answer is not good enough. Where does this number come from? How old is the research? Surely, we have new a better numbers to give to our patients. I am not sure that we will have a new or better study. The quoted risk associated with amniocentesis comes from studies done in the 1970’s. This predates routine use of ultrasound for amniocentesis (just push the baby up and aim over the bladder, as my preceptor taught me), and included women having procedures for a variety of reasons. One of the more common reasons in the study was investigating pregnancies affected by Rh isoimmunisation, a condition that is now routinely monitored with ultrasound and amniocentesis is used more for treatment, when intrauterine transfusions are indicated. This is not the same distribution of pregnancies requiring amniocentesis nowadays.
There are newer studies, but they are limited by being retrospective in nature. It is not actually ethical to randomize women to having or not having an amniocentesis. The appropriate control group is difficult to define. Many women do not realized that there is a baseline 1-3% pregnancy loss rate after fifteen weeks. The additional risk of the amniocentesis must also take into consideration the indication for the procedure. Fetuses with abnormal maternal serum markers are at higher risk for pregnancy complications, even without an amniocentesis. Fetuses with abnormal chromosome counts are at a higher risk for spontaneous loss, as are those fetuses with multiple anomalies. This is what makes it so difficult to find an appropriate control group. There may be more pregnancy losses in a group of women who undergo amniocentesis, but these women were already selected because they have a higher risk to their pregnancy.
Patients also want to know what is my personal risk associated with amniocentesis. I actually don’t know. I have had twins deliver early after performing a reduction, but the demised twin had hydrops and polyhydramnios. Was the amniocentesis the risk factor, or was it just the risk of twins, or worsened by the polyhydramnios. I will never know. I do feel bad that the surviving twin was born at 25 weeks, but I am not sure that my actions directly contributed – it was already a very high risk pregnancy, and a good outcome was not a guarantee. I understand that my numbers are tracked by the province, but I do not have direct access to them. I am curious if I can request this information, or if patients can before meeting me for the procedure. I would be a little hesitant though, because I know I have a lost rate after amniocentesis because I am one of the physician who performs genetic terminations, and I am not sure how that would be reflected in my numbers.
An amniocentesis, like any other medical test, has risks and benefits. I cannot decide the right choice for my patients. From a screening standpoint, I like the risk of 1 in 200 because it reflects the risks on screening tests after which amniocentesis is offered. I suspect the actual risk is lower. The risk of 1 in 200 is also the most quoted, so I am not going to off and say that I am so much better that average, sometimes just a little dumb luck can affect the outcomes. I believe that each pregnancy comes with its own risks. Sometimes I will offer an amniocentesis, sometimes I will recommend one and other times I recommend against the procedures. Studies generally apply to population risk, and part of the art of medicine is to translate it into an individual risk for each patient.