Preimplantation genetic diagnosis (PGD) is a diagnostic test for the detection of chromosomal abnormalities or single gene defects in the embryo before the embryo(s) are transferred back into the uterus during In Vitro Fertilization (IVF). The test is most often utilized in couples with known genetic mutations such as Cystic Fibrosis (CF) as a way to prevent transmitting the abnormal gene to their children. New technological advances allow for rapid detection of chromosomal or single gene defects in the embryos by doing the embryo biopsy at the blastocyst stage (5 days after the eggs are fertilized).
How is PGD done?
Embryo biopsy utilizes sophisticated microscopic laser technology to remove few peripheral cells (trophectoderm cells), which are destined to become the placenta. The cells destined to become the fetus are not disturbed. The cells are carefully dissected with pulses of laser energy and sent to genetic lab for analysis by Comparative Genomic Hybridization (array-CGH). Only those embryos which are found to be normal are transferred into the uterus. The abnormal embryos are discarded. Once this technology becomes more cost effective, all embryos may be routinely biopsied to avoid transfer of abnormal embryos.
What is PGS?
Routine genetic screening of all embryos during IVF is called Preimplantation Genetic Screening (PGS). It involves biopsy of all embryos reaching the blastocyst stage during IVF to determine which embryos are genetically normal. Older screening technologies for abnormal embryos were found to be inaccurate. Typically, during IVF, 1 or 2 embryos are transferred into the uterus at the blastocyst stage. Not all embryos progress to the blastocyst stage and not all blastocysts are genetically normal. Selecting the right embryo for transfer is tricky because looks can be deceiving. The embryo may look good but still be genetically abnormal. Remarkably, about 40% of chromosomally abnormal embryos can reach the blastocyst stage. Not surprisingly, when a single unbiopsied blastocyst is transferred, the ongoing pregnancy rate in patients under the age of 35 is only about 40%.
Ideally, only one embryo should be transferred to minimize the risk of twin pregnancy. Newer technologies such as array Comparative Genomic Hybridization (array-CGH) have been utilized to accurately analyze the chromosomal content of the embryos and help choose the right embryo for transfer. In some studies, when a single, biopsy proven, genetically normal blastocyst is transferred, the ongoing pregnancy rate is markedly increased. The current trend in IVF is to biopsy all blastocysts and to transfer a single, biopsy proven, genetically normal embryo. Another option is to freeze all genetically normal embryos for transfer in a non stimulated cycle. Both strategies have shown an improvement in the reproductive outcome for patients undergoing IVF.
To learn more about PGD at Boca Fertility give us a call at 1.844.207.0044