COMPARATIVE GENOMIC HYBRIDIZATION (CGH)
A newer technology called comparative genomic hybridization (CGH) allows testing for all 46 chromosomes. The limitations exist as with other types of preimplantation genetic tests (PGD) including SNP, array CGH and FISH from the perspective of mosaicism, which may produce false positive and false negative results. In other words, an embryo can be labeled as being normal, while it’s abnormal or the other way around. Mosaicism has been reported to range from 5-10% in embryos.
The benefit of CGH is its ability to diagnose any numerical abnormality (aneuploidy) in the embryo because it tests for the full 23 pairs (46 chromosomes). The pregnancy rates achieved with this technology appears to be 60-70% per cycle based on medical studies. It is also an interesting finding that the miscarriage rate is very low (~5%). This is of no surprise because most miscarriages are due to chromosomal abnormalities of the embryos which can be diagnosed by CGH, and only chromosomally normal embryos can be transferred into the uterus.
Routine prenatal care and prenatal genetic screening should be recommended in cases where genetic testing was done on the embryos during IVF treatment, including 1st trimester chorionic villus sampling (CVS) and/or 2nd trimester amniocentesis. More recently, non-invasive first trimester screening has been implemented into routine pregnancy care and may be a helpful tool. Ultrasound evaluation of risks in the second trimester along with hormone testing is also recommended.
Currently, none of the preimplantation genetic screening tests is 100% reliable or accurate, but do provide helpful information that can be used in the clinical setting. Dr. Bayrak recommends discussing the risks, benefits and alternatives of each genetic screening test with patients based on individual cases before starting treatment.
