Prenatal Genetic Testing  and the Dangers of Amniocentesis

Today I would like to talk about Prenatal Genetic Testing. This is an important part of Prenatal Care. 

When a woman is pregnant, or wants to be soon, I will provide what is called "Prenatal Care". This type of care is provided by an Obstetrician. This job is what makes an Obstetrician an Obstetrician. Ob's are usually gynecologists as well, but there are many Obstetric sub-specialists in Maternal Fetal Medicine who are not gynecologists.   They only provide care to pregnant women. MFM's provide an important part of prental care, but they usually don't provide the whole spectrum of prenatal care from pre-conception to post-partum. As far as myself, an obstetrician, I provide pre-natal care, as well as take care of high risk pregnancies. Personally, I don't hesitate to call in the MFM's when their consultation is appropriate. 

Prenatal care is important for many reasons. Prenatal care has been shown to reduce obstetric morbidity and mortality about 99 percent. That is a huge reduction in risk to the new mom. Another way of looking at it, is there is a 99 fold reduction in risk. Women who are pregnant without any prenatal care are putting themselves and their babies in a very risky situation. 

One important part of prenatal care is evaluating the genetics of the fetus. One of the most common genetic birth abnormalities is Down Syndrome. For a 20 year old, the risk of having a Downs baby is about 1 in 1000. For a 30 year old, about 1 in 500. For a 35 year old, the risk is about 1 in 300. For a 40 year old, about one is 50. It is higher yet for older women. 

The historical medical test to screen for Downs Syndrom is called Amniocentesis. Amniocentesis consists of putting a sharp hollow needle near to the baby, through the mothers abdomen, into the babies amniotic sac, and withdrawing some of the amniotic fluid. We usually take 20 cc's. This is a very small part of the total.  Amniotic fluid is mainly the fluid that is created in the babies kidneys, and the baby urinates it out into the sac. Amniotic fluid is very similar to urine, but it doesn't have urine smell, and it is almost never yellow. Amniocentesis is very easy to do if an Obstetrician is well trained. We always use the sonogram to guide the needle placement, to keep the needle away from the babies vital parts, and to avoid the umbilical cord. I have done this procedure more times than I can count. After the amnio, we get a preliminary FISH chromosomal analysis in about 2-3 days. 

The problem with amniocentesis is that it has some significant dangers. About 1 percent of the patients that have this done will have a persistent leak of amniotic fluid from the vagina. This is frightening, but it usually heals up in a matter of hours. Unfortunately, some babies will miscarry after the amnio. The studies that measure the risk are all over the map, but the reported risk varies from 1 percent, or 1 in a hundred, to 1 in 1500. If the miscarriage happens to you, then the statistics mean nothing. It is just painful.  

Because the Amnio is risky, Ob's usually restricted it to women who had some kind of elevated risk. Most commonly, the elevated risk is what we call Advanced Maternal Age, or 35 years old at delivery. That would be 34 and 1/4 years of age at conception. Other risk factors would be a family history of chromosomal problems, including any past miscarriages with chromosomal abnormalities. 

Women really hate having to think about having an amnio. They resist it quit a bit. In my practice, if I detect that a woman is simply afraid of the needle, then I will try to counsel her that the needle pain is small and temporary, and insignificant compared to the importance of the knowledge gained. 

But the good news is amnios are no longer needed for screening. We now have a newer nonivasive technology call a nuchal test. In this test, we do a sonogram at 12 weeks and look for a couple of important markers of fetal well being. Those markers are the nasal bone and the nuchal fold thickness. We then test for some biochemical markers of fetal well being. This nuchal testing technology has no direct risk to the baby. The nuchal test will not give us a yes or no answer about Downs Syndrome, but it comes close. At 12 weeks we get a test result that shows odds against Downs. Many times the odds go below 1 in 10,000. 

Then we do the sequential test at 16 weeks. This is only blood work. After that the odds against Downs Syndrome can go to as low as 1 in 250,000. When a pregnant women gets a report like this, she will have to think long and hard if she still wants to risk the Amnio. This is because the amnio might cause thousands of miscarriages of healthy babies before it was able to detect a downs baby. I believe that an amnio remains a right for pregnant women, so she should have it if she wants it, even in low risk scenarios. But it is very rare for a woman to choose it if the risks of Downs are this low. 

On the other hand, if the nuchal and/or sequential test shows an elevated risk for Downs, then we should offer the amnio to the woman. Some women will do it, some won't. A common reason for not doing it is that the woman and her family have religious objections. Those families will never terminate any baby. Even one with downs syndrome. These women should be counselled that the amnio is unnecessary. Sure, we might gain some information, but we would be risking the normal and healthy babies to get at that knowledge. For those  women, it is important that they make choices that are consistent with their spiritual values and needs. It is my job as an obstetrician to help her make choices within her spiritual needs. If the woman would never terminate a Downs baby, then they should not choose to risk all the healthy ones by doing amnios. 

Thanks everyone for reading my blog. 

There is tons of things to write about. 

I have a lot of ideas in mind. 

Sincerely, 

Dr. John Marcus MD 

Obstetrician and Gynecologist 

Ridgewood, NJ, 07450 

201-447-0077