Preimplantation Genetic Screening (PGS)

The nucleus of every cell in the human body contains 23 pairs of chromosomes, or 46 total chromosomes.  Each set of 23 chromosomes is inherited from each parent (23 from the father and 23 from the mother).  When an aberrant number of chromosomes exists in a fetus, less than or greater than 46 chromosomes, aneuploidy exists.  Down syndrome is an example of aneuploidy, where an extra chromosome 21 was inherited from one of the parents (typically the mom).  The consequences of aneuploidy are often serious, with most aneuploid embryos failing to implant or miscarrying.  Occasionally, as in the case of Down syndrome, an embryo may develop and deliver at term, with significant mental and physical impairment.  Preimplantation genetic screening (PGS) is a technique similar to PGD, which allows determination of the chromosomal make-up of an embryo prior to embryo transfer into the uterus.  In PGS, one or two cells (blastomeres) are removed from a 3-day old embryo for genetic analysis.  Then, a technique called fluorescent in-situ hybridization (FISH) is employed to determine how many copies of a particular chromosome exist within each embryo.  For instance, if FISH is performed for chromosome 21, and 3 copies are found, the presumed diagnosis of trisomy 21 (Down syndrome) is made, and that embryo is excluded from transfer.  Each chromosome tested should have two copies only.

The most common cause for miscarriages is aneuploidy, and the most common cause for aneuploidy is advanced maternal age.  As such, in couples with prior miscarriages and in women with advanced reproductive age, PGS has been employed prior to embryo transfer to reduce the likelihood of miscarriage.  PGS has also been employed with the theoretic advantage of improving implantation and pregnancy rates following IVF and reducing the likelihood of delivery of an anomalous child (such as a Down syndrome affected child) in older women.  Several clinical trials have illustrated such benefits with PGS, while others have not.  Additional trials are needed to better define the utility of PGS in cases of recurrent miscarriages and advanced maternal age. 

PGS has been conclusively proven to be useful for decreasing the likelihood of miscarriage in couples with habitual abortions determined to be due to a specific chromosomal abnormality in one of the parents, such as a Robertsonian or reciprocal chromosomal translocation.  When a translocation exists in one of the parents, two different chromosomes are attached to one another.  Despite the fact that the parent carrying the translocation is completely normal, this abnormality leads to the production of gametes (eggs or sperm), which are often chromosomally abnormal (missing a particular chromosome or having an extra chromosome).  This often results in a chromosomally abnormal fetus and/or recurrent miscarriages.  In PGS, specific chromosomal probes are developed for the translocation in question, and chromosomally abnormal embryos are excluded from transfer.  This leads to a dramatic reduction in the risk for a chromosomal abnormality in the fetus and miscarriage. 
  
Commonly, the chromosomes tested for in PGS are those known to be involved in miscarriages and those which may lead to the birth of an anomalous fetus (with congenital birth defects).  Various chromosomal panels may be applied, the most common of which includes chromosomes 13, 15, 16, 18, 21, 22, X, and Y.  However, any chromosome may be tested according to the specific chromosomal abnormalities that may exist in one of the parents.

The California Center for Reproductive Health is pleased to offer PGS to couples in need.  Only safe and proven biopsy techniques are employed to ensure that embryos remain unharmed.  Analysis of the chromosomal make-up of each biopsied embryo is performed by expert embryologists with outmost precision and accuracy to ensure effective preimplantation screening.