Infertility Fast Facts
Couples approximately in the United States, or 10 percent of all couples of childbearing age, have difficulty conceiving.
Preimplantation Genetic Diagnosis (PGD) and Preimplantation Genetic Screening (PGS) Options for IVF
Typically, embryos are observed under the microscope and our embryologist visually selects the best or highest quality embryos for transfer into the patient’s uterus. Embryologists grade embryos based upon several visually observable characteristics. These characteristic include: cell divisions, cell size, and the fragmentation of the embryos. Once the embryologist looks at each embryo they are given a letter and numeric grade, or score to indicate apparent normal makeup. These “scores” are used to determine which embryos appear to be normal and which appear to be abnormal. Unfortunately, embryologists are not able to determine the genetic makeup of the embryos just by looking at them. Although most IVF cases are fine with visualization of the embryos in some cases where there is a known or suspected genetic family illness more advanced embryo screening may be warranted.
Preimplantation Genetic Diagnosis (PGD), or Preimplantation Genetic Screening (PGS), is a test used in conjunction with IVF to help couples determine the exact genetic makeup of the embryos before the IVF transfer occurs. This testing helps increase the chances that a chromosomally normal embryo is selected for transfer and will result in a successful pregnancy.
PGD and PGS testing are used with IVF, in vitro fertilization
PGD is performed three days after the egg retrieval takes place. The embryologist biopsies, or removes, one cell from each of the couples embryos and sends these cells to a genetic laboratory for the necessary testing and chromosomal analysis. Results from the PGD/PGS testing are received on day 5, just before the embryo transfer takes place. PGD results tell the embryologist which of the biopsied embryos are chromosomally normal and which are not normal. The results will also tell what type of genetic error has occurred in each of the abnormal embryos.
Not every couple going through IVF will need PGD.
Usually, Dr. Brown recommends PGD when one of the partners is a carrier for a gene that can cause disease or one of the partners suffers from a genetic disorder that a couple does not want to pass on to their offspring. Cystic Fibrosis, Spinal Muscular Atrophy, Sickle Cell disease, and Tay-Sachs are just a few of the diseases PGD can help to identify.
Testing for a known single gene defect is now possible at Reproductive Institute of South Texas.
If testing for a specific genetic disorder, or single gene disorder, the genetics laboratory must first develop a probe or screening tool that is specific to the individual couple’s DNA. This screening tool is used to test each embryo from an IVF procedure for the specific genetic disease. Probe development can take 4 to 12 weeks or more depending on the information the lab needs to obtain. For example, the lab may use blood and saliva DNA samples from each parent to gather the data required for the probe.
After development, the probe is used to compare the individual embryos’ DNA to the DNA of the parents to look for the specific mutation that causes disease. This ensures that the known defective gene is not passed on to the couple’s children. The IVF cycle will begin once the lab has created the probe to screen the embryos and the results from the genetic analysis have been given to the embryologists and Dr Brown for review.
PGD for Known Chromosomal Translocations
Dr. Brown may also recommend PGD when a patient has a balanced translocation of genetic material present in their own genetic code. A balanced translocation occurs when a given amount of chromosomal information on one gene is switched with an even amount of genetic material on another gene. The chromosome remains fully functional in the adult but can cause problems during the fertilization process between the egg and sperm as the chromosomes may become unbalanced. Unbalanced chromosomes often result in recurrent miscarriages because the baby cannot survive with the mixed up genetic material.
The individual, either the male or female partner, with a balanced translocation is genetically normal, but may have trouble producing a normal baby due to this mix up in genetic material. Unbalanced translocations cause the chromosomes to receive unequal amounts of information and result in extra or missing genes in the embryo. This is not usually compatible with life and the pregnancy ends in a miscarriage.
Dr. Brown can use PGD to determine which embryos have unbalanced translocations prior to implantation, and can transfer the healthy balanced embryos back into the uterus for a successful pregnancy.
Couples do not have to have a specific disease or chromosomal abnormality to utilize PGD/ PGS. Often, Preimplantation Genetic Screening, PGS provides valuable information for couples who have experienced recurrent losses, women of advanced maternal age, and patients who have had repeated IVF failures.
Chromosomal abnormalities are the most common cause of early miscarriage, and PGS enables the fertility specialists to select chromosomally normal embryos for transfer into the uterus.