PGD and PGS are laboratory techniques used to evaluate embryos before they are transferred back into the uterus.
Both techniques are performed in conjunction with an IVF cycle. Embryos formed as a result of IVF can be tested for chromosomal abnormalities or genetic diseases prior to embryo transfer and in order to decrease the chance of finding a chromosomal problem or diseased fetus during prenatal testing using chorionic villus sampling, amniocentesis or fetal blood cell testing. These techniques can assist in the selection of the embryo that is most likely going to result in the establishment of a pregnancy and a healthy baby. Embryo testing is often done at the blastocyst stage (day 5 of development) via trophectoderm biopsy (biopsy of placental cells) for the most accurate results. Because of the length of time required for these tests, day 5 embryo testing necessitates that the embryos be frozen immediately before the results are back, and then the normal embryos are transferred back during the next menstrual cycle.
Preimplantation Genetic Diagnosis (PGD)
With PGD the embryos are tested for specific diseases, such as cystic fibrosis, beta thalassemia and retinoblastoma. Sex linked diseases such as hemophilia can also be identified. Testing can also be performed for human leukocyte antigen (HLA) matching and chromosomal translocations. Embryos can be screened for mutations that increase the chance of disease, like the cancer predisposition mutation BRCA1. Most diseases and disorders that are the result of a genetic mutation can be tested for via PGD. Depending on the mutation being tested for, either a protocol may already be in place or a new protocol will have to be developed in order to identify the embryos that carry that specific mutation.
Preimplantation Genetic Screening (PGS)
Women of advanced maternal age or women with a history of recurrent miscarriage may be more likely to have embryos that are chromosomally abnormal. PGS can identify those embryos that have a gain or loss of genetic material (aneuploidy). Aneuploid embryos will most often not implant in the uterus after transfer (leading to a negative pregnancy test), however they are the most common cause of miscarriages in the first trimester and can sometimes lead to the birth of a child with severe health issues. Like PGD, PGS is also performed on day 5 of development (see PGD section) and therefore the embryos are frozen and placed back in the uterus in a subsequent cycle. The screening is performed with technology that allows for testing of all 23 pairs of chromosomes.