Preimplantation genetic diagnosis (PGD) is being used increasingly due to dramatically improve the chance of a successful IVF pregnancy in couples where prior IVF failures have remained unexplained. It has been estimated that over half of iBaby Fertility failures are not able to be explained by an apparent problem with embryo “quality”. Generally, embryos are given “good” marks when they demonstrate an appropriate number and equality of cell divisions at a given time in their growth cycle, when the individual cells of the embryo appear to have a uniform size and when there is an absence of cellular “fragments” that may or may not represent problems in the growth progress of the embryo.
PGD has offered physicians and scientists, for the first time ever, the ability to examine far beyond the superficial appearance of an embryo. We are now able to examine the most important internal genetic code of the embryo as well. And with these new genetic tools, we have come to learn that some embryos that appear on the surface to be of the highest quality may carry a genetic code that makes them poor choices for attempting to establish a healthy pregnancy. In addition, Pre-implantation Genetic Diagnosis (PGD) is a laboratory procedure, used in conjunction with IVF, which helps to reduce the risk of passing on inherited conditions. Some of the most common reasons for PGD are specific single gene conditions (such as cystic fibrosis or sickle cell anemia) or structural changes of a parent’s chromosomes. Families may also use PGD when a member of the family is in need of a bone marrow donor, as a way to have a child who can provide matching stem cells.
HOW IS PGD WORKS PGD utilizes IVF, where multiple eggs are matured and retrieved; the oocytes are inseminated with sperm and the resulting embryos are grown in culture until the 6-8 cell stage (day 3 of embryo development). At this point, the embryo is biopsied with the removal of 1-2 cells. This process does not damage the cells remaining within the embryo. The isolated cells are evaluated for the specific genetic condition anticipated. Embryos that are determined to be unaffected are transferred back to the woman’s uterus on day 5 of embryo development.
Two main techniques are used for the genetic assessment: Polymerase Chain Reaction (PCR) and Fluorescent in Situ Hybridization (FISH). In PCR, multiple copies of the gene of interest are made by a process of amplification. This amplification process allows the identification of very small amounts of DNA in order to make the diagnosis. FISH allows the laboratory to actually count the number of chromosomes within the isolated cell. This technique is utilized primarily for expected abnormalities in chromosome number (e.g. trisomy – three copies of – 21 or Down Syndrome) or translocations (defects in the structure of the chromosome).
PGD for Aneuploidy Screening (PGS)
In an genetically normal embryo, there should be 2 (and only 2) chromosomes of each of 22 chromosome types in addition to one set of XX female or one set of the XY male chromosomes. Any more or less than 2 will result in defective embryos. This condition is called “aneuploidy” and will result in an embryo that will fail to develop and implant or will miscarry or may even produce a defective baby. When assisting couples that have failed IVF in other programs, PGD allows us to go an important step beyond what has been done in the past, and identify and separate the chromosomally normal embryos from those “abnormals” that have no chance of leading to healthy pregnancy. Armed with this crucial genetic information, we can assure with quite reasonable certainty that those embryos that truly provide a chance of healthy pregnancy are the ones returned to the uterus.
Who is indicated for PGS
– Prior unexplained IVF failure
– Recurrent miscarriages
– age over 35 years old
– Previous conception of a chromosomally abnormal child or fetus
– Polycystic ovary syndrome
– History of ovarian hyperstimulation syndrome
– Heritable medical condition in either the patient or in a prior child (such as hemophilia)
– Two or more children of the same gender (sex selection for family balancing)
– Multiple birth with desire to transfer just one embryo or a need for donor oocytes due to Turner’s syndrome
– Premature ovarian failure or premature menopause
PGD (Pre-implantation Genetic Diagnosis)
PGD involves the genetic evaluation of early stage embryos that have been produced through in-vitro fertilization (IVF). PGD allows us to determine both sex of individual embryos (female or male) and their chromosomal abnormality. There is no evidence that PGD leads to an increase in birth defects or chromosomal disorders. PGD is done before the embryo’s genetic material becomes ‘active’ when the cells inside the embryo are still all identical and each cell is still capable of becoming any part of a baby. Removal of one or two cells from the early embryo does not alter the ability of that embryo to develop into a complete, normal pregnancy.PGD is more than 99% accurate so is the perfect tool for patients wishing to choose the gender of their baby.
The PGD process
Embryos produced in an IVF cycle are cultured in the laboratory for 3 days when they will have developed approximately 6-8 cells. Embryos with normal development on Day 3 will have one or two cells removed for testing. This is performed by placing the embryo under a powerful microscope and using a Laser to make a small cut in the zona pellucida, (a tough outer membrane holding the embryo together). Depending on the health and size of the embryo, one or two cells inside are taken out.
How to select the embryo
PGD testing is different to most genetic testing, since it is performed on only one or two embryonic cells and must be completed within 48 hours to allow embryo transfer by Day 5. Since standard chromosome analysis takes several days, a different method called Fluorescence In-Situ Hybridization (FISH) is performed that can be completed within 4 to 6 hours. Each chromosome has unique areas of DNA present only on that chromosome. A small DNA probe is used to recognize these unique patterns and lights up when it attaches to the chromosome. Each probe shines light in a different color, allowing several chromosomes to be tested at the same time. This technique is known as FISH.
Selecting embryos to transfer
We only transfer embryos that have a normal test result and appearance. The combination of normal genetics with normal physical appearance gives each embryo the best chance of developing into a healthy pregnancy.
Dr. Pisit and embryologist team will make decision together which embryos should be transferred. The remaining embryo will be frozen and stored in a Liquid Nitrogen Tank. You also can also ‘balance’ or complete your family with embryos of the opposite sex if you have a successful first pregnancy and want another child later on.