Thursday, May 23, 2013

Screening for Chromosomal and Genetic Disorders

Most of the testing we offer during prenatal care is routine, but certain testing is optional.  Specifically those tests include testing for chromosomal disorders in the fetus and for genetic carrier status in the parents.  Chromosomal screening tests have evolved significantly over the years, starting with second trimester (15--20wks) screening (blood test) and now to first trimester screening including a blood test and ultrasound for the NT (nuchal translucency=the thickness of the skin fold on the back of the baby's neck) done at 12--13 weeks.  For older moms (> or = 35) there is a new test that measures the fetal DNA in the mother's blood as well ("Panorama" testing).  The main abnormalities that can be determined are Trisomy 21/Down's syndrome, Trisomy 18/Edward's syndrome and Trisomy 13/Patou syndrome.  Panorama testing can also pick up Monsomy X/Turner' Syndrome.  The only tests that can measure all the chromosomes are Chorionic Villous Sampling (CVS) or Amniocentesis, both of which are slightly invasive and can be associated with a small risk to the pregnancy/fetus.  They are therefore only offered when the risk is thought to be rather high that a disorder is present.  The majority of chromosomally abnormal fetuses will miscarry spontaneously, but these few disorders have occurred with enough frequency in liveborn infants to allow tests to be developed.

Genetic carrier screening can be done pre-conceptually or after conception of a pregnancy.  The most familiar example of such a test would be Cystic Fibrosis screening, which has been available for a number of decades.  As an example, 1/29 Caucasian people are carriers and the random chance a baby could be born would be about 1/3000.  However, if either the mother or father were to screen negative for the 20--40 most common genetic mutations associated with the disorder, the risk decreases to 1 in 6 million.  Other genetic tests have been available for a number of years including a panel of diseases for which Ashkenazi Jewish people are at risk, a form of mental retardation called Fragile X, and a muscular disorder called Spinal Muscular Atrophy.  A few companies have also come up with a much more comprehensive panel of disorders that can be screened for as well.  Many are quite rare, but for those families who have an affected child, most say they wish they'd had some information ahead of time about the disorder.  Some disorders can be treated while others cannot.  Genetic counseling is provided to those families in whom carrier status is found to exist in BOTH parents (with the exception of Fragile X, which only requires one parent be affected).  More information on this can be found at:

Many patients will ask why to do screening tests if they don't intend to terminate an abnormal pregnancy.  My answer to this question is that while there is no cure for many of the conditions for which we screen, there are a number of ways that it would change the way in which we care for you during your pregnancy should we discover certain conditions in your baby.  For example, when I'm taking care of a fetus with Down's syndrome, I'll order a special ultrasound of the fetal heart called a fetal echocardiogram  around 22 weeks of pregnancy to look for defects that are more common in these babies.  I'd also follow the fetal growth and amniotic fluid volume more closely in the third trimester to look for signs of problems that could put the baby at a higher risk of stillbirth.  Other specialists are also sometimes consulted for assistance in planning care for the baby immediately after birth or in the weeks and months thereafter.

All that being said, we have many patients who opt out of all or most of this testing, and that is perfectly OK with all of us.  We just like to make sure that our patients are kept fully apprised of what is currently available in order to be able to make a thoughtful and educated decision regarding their prenatal options.  

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