Last week an article by The Associated Press that appeared in newspapers across the country caused concern in religious and ethical communities as prenatal genetic testing was linked to abortions as a means of selective breeding to remove unwanted traits from the population. The article cited an unsourced California study that concluded prenatal screening reduced by half the number of babies born with the severest form of cystic fibrosis because many parents chose abortion.
The article stated, "Births of babies with cystic fibrosis, Tay-Sachs and other less familiar disorders seem to have dropped since testing came into wider use, The Associated Press found from interviews with numerous geneticists and other experts and a review of the limited research available."
While it is easy to understand why health care providers might welcome a reduction in the number of patients they treat with serious genetic-based illnesses, no couple wants to be in the difficult position of choosing whether or not to terminate a pregnancy after they find that they are pregnant with a fetus with a serious genetic illness that will experience significant suffering and a shortened life if the pregnancy proceeds to a birth.
But there is another option. In most instances, the AP article was referring to the genetic testing of fluid collected during routine amniocentesis conducted as a part of an expectant woman's prenatal care during the 16th to 18th week of pregnancy.
There is another form of genetic testing available, just as reliable and just as thorough as an amniocentesis, but that occurs months earlier, even before the pregnancy begins, in an IVF laboratory -- one that is associated with far fewer ethical issues.
Preimplantation Genetic Diagnosis (PGD) is a technique in IVF where, prior to an embryo being transferred into an intended parent, its genetic health is analyzed by closely examining its chromosomes. Since most intended parents undergoing an IVF cycle have multiple embryos from which to choose, the purpose of PGD is to transfer only the ones that are chromosomally normal, or without recognizable genetic disorders.
The basic science of PGD
The following is the shortest biochemistry lesson in history.
Chromosomes are tiny threadlike structures composed of DNA and protein found in the nucleus of cells. They contain the genetic material (genes) that determine the individual characteristics of an organism and control body functions. Normal body cells of humans contain 46 chromosomes (22 matched pairs numbered: 1, 2, 3, 4 ... up to 22 and, one set of sex chromosomes, XX or XY). One set of chromosomes is received from the egg and the other from the sperm.
PGD is performed during the early stages of embryonic development, by carefully removing one of the cells found inside an embryo and examining the chromosomal material.
PGD for chromosomal abnormalities
Abnormalities in the number or structure of chromosomes are often present during embryonic development, which can translate to abnormalities in the individual. Having an extra or missing chromosome(s) (described as aneuploidy) or fragments of chromosomes in the wrong places, can have serious consequences, including the arrest of embryonic development and failure of implantation and pregnancy. If a pregnancy does occur, the rates of miscarriage are higher than the norm. If the baby makes it to term, the child will have a serious genetic abnormality that is probably life-threatening.
As women age, their eggs become more susceptible to chromosomal defects and aneuploidy rates increase, causing a lower overall pregnancy rate, more miscarriages and a greater rate of abnormalities in their offspring.
PGD for specific genetic disorders
PGD has also been successfully applied to testing for single-gene disorders in couples at risk of transmitting a genetic disease to their offspring. The range of inherited diseases for which PGD has been applied in the last decade has extended to more than 100 different conditions, with the most frequent ones being cystic fibrosis and hemoglobin disorders. Knowledge of the specific DNA sequence involved in the gene mutation is necessary; thus for each couple, preparatory work is required.
Who opts For PGD
1. Women over 35. As women age, egg quality often declines, increasing the risk of miscarriages and chromosomally abnormal embryos.
2. Couples who are chromosomally normal but have a history of unexplained, repeated miscarriages.
3. Couples who have had repeated, unexplained IVF failures.
4. Patients using a first-time egg donor or an egg donor that has a history of poor embryo quality.
5. Couples at risk of having a baby with a genetic disorder.
The Reproductive Sciences Center in La Jolla is the only fertility center in San Diego that has an in-house PGD facility. While it is a complex procedure, in the hands of experienced laboratory staff, it is safe, effective and a reliable way for intended parents to get some peace of mind.
Jimenez is director of corporate communications at Stemagen. Wood, M.D., Ph.D., is medical director at the Reproductive Sciences Center in La Jolla.