Genetic Screening


In an earlier post I alluded to the desire for 'the perfect child'. This morning, on NPR's Morning Edition, "NPR's Jon Hamilton reports a genetic test for cystic fibrosis in a fetus is raising big questions about making such tests widely available. Some doctors and patients may be misunderstanding the test results, and that could be leading to unnecessary abortions. " ( Will be rebroadcast between 0930 and 1000 EST and 0830 and 0900 PST, and will be available on the web site later today).
18 months ago, the American College of Ob/Gyn issued a recommendation that all pregnant couples be offered screening to see if the couple is at risk for having conceived a child with cystic fibrosis. This screening recommendation is just the latest in a series of like recommendations. It started in the late 1970s with the recommendation that women over the age of 40 (later dropped to 35) be offered amniocentesis and karyotyping (chromosomal analysis) since the incidence of all chromosomal abnormalities increases (as a percentage of pregnancies) as the woman ages. Numerically, more chromosomally abnormal children are actually born to younger women, but the likelihood of a specific pregnancy being affected does increase with age. Amniocentesis is not a benign procedure. Although considered relatively 'safe' it does invade the uterus and can trigger bleeding and in rare cases premature birth or miscarriage. The anxiety while waiting for the results to return is also not inconsiderable. An excellent book on this topic is The Tentative Pregnancy by Barbara Katz Rothman.
The basic assumption behind this, and later tests, is that mothers and fathers want to know and want the option of abortion if they conceive a 'less than perfect' child. It is also being offered on the basis of making decisions about place of birth and post-birth options such as surgery, therapy, etc. But the basic idea is to prevent the birth of babies with various handicaps and disabilities, and it has been very successful. The March of Dimes has led the campaign to make more and more prenatal screening available and to make it a standard of care that midwives and doctors ignore at their peril.
We are now required to offer testing to every pregnant woman. Maternal serum screening is a blood test done on the mother between 15 and 20 weeks of pregnancy. It looks at levels of various chemicals (alpha feto-protein, estriol, human chorionic gonadotropin and possibly others) in the mother, and compares the level of these chemicals to the statistical norms across the population to estimate the risk that the baby will have one of several conditions. This test requires accurate dating of the pregnancy and an accurate weight on the mother on the date the blood is drawn. It is NOT diagnostic. It simply identifies a sub-group of women and babies for whom additional testing is recommended. The vast majority of 'bad news' on this test turns out to be a false alarm - meaning that the further testing done cannot identify a specific problem. The further testing usually involves an extensive ultrasound examination to accurately date the pregnancy and to look for visible defects. Sometimes amniocentesis is recommended also.
Most people expect to have one or more ultrasound examinations during the course of pregnancy. This window into the womb is fascinating, and can personalize the baby to the mother. While generally considered non-invasive, it has only been widely available for 20 some years and the technology is continually evolving. Interpretation of the images requires skill especially when trying to identify some of the smaller body parts. Also, certain findings are appearing that have uncertain significance. Not every birth defect can be picked up US, nor does every abnormal finding on an US turn into a birth defect.
Some kinds of genetic screening are targeted to specific ethnic groups. Examples include sickle cell, thalassemia, Tay-Sachs, and a host of other conditions found in certain families or groups. Most of these conditions are what is called autosomal recessive - meaning that in order to have the condition, one would have to be given the gene from each parent. In order to transmit the condition, both parent would need to be carriers (having the gene but not the condition). If both parents are carriers, the chance for each pregnancy of having an affected child is 1 in 4. If one parent has the condition and the other does not and is not a carrier, all children will be carriers, but none will have the condition. If one parent has the condition, and the other parent is a carrier, all children will be carriers, and the chance for any pregnancy to be affected in 1 in 2. If both parents have the condition, all children will be affected. (autosomal dominant and mixed are different cases and I won't go into them).
What makes screening for cystic fibrosis and a few other conditions more difficult is that there are many variations on the genes that can cause this disease. The testing currently available looks for 25 of these mutations. In the ethnic group at highest risk (caucasions of European ancestry) it is possible to identify close to 90% of actual carriers, but the rate goes down to 70% or so in other groups).
It sounds so simple, actually. You test the mother - if she is a carrier, then you test the father. If he is also a carrier, you tell the mom that the baby has a 1 in 4 chance of having CF and you offer amniocentesis. But what sounds so simple becomes incredibly complex, given that we are dealing with human beings. And how does the prenatal diagnosis affect the entire experience of the pregnancy? Suppose that you are told that your child has this one in four chance - what do you do? Do you go for the added testing? Do you chew hour nails to the bone for the rest of the pregnancy? Do you panic every time your newborn has the sniffles or a cold? What do you do?
And if you decline testing, and you end up with a baby with a problem that could heve been detected, what do you do then? How do you feel?
We have let the genie out of the bottle. We are still just at the very beginning of the ability to do prenatal diagnosis. Most conditions detected cannot be treated, if at all, until after birth. Knowing about some of these conditions does improve outcome - where immediate post-birth surgery is helpful, birth can be planned to take place near a facility with the right people and equipment. In very rare cases, fetal surgery can be an option. Knowing ahead can help to prepare for helpful interventions. But very often, the first response may be to end the pregnancy, to 'take care of the problem'. These are often the cases where 'partial birth abortion' is chosen - as diagnosis is usually between 20 and 24 weeks of pregnancy.

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This page contains a single entry by alicia published on April 4, 2003 7:46 AM.

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