Preimplantation Genetic Diagnosis
PGD for Chromosome Translocations
Chromosomes contain all of the genetic information that determines everything from the color of our hair to the way our body functions. A normal cell in the human body contains 46 chromosomes in 23 pairs. We inherit one chromosome of each pair from our mother and the other chromosome from our father. Females have two X chromosomes, while males have one X and one Y chromosome. Each egg cell and sperm cell contain 23 chromosomes and when they come together at fertilization the embryo will then have 46 chromosomes.
Chromosome analysis is often performed on parents who either have a history of miscarriages or a child with a chromosome abnormality. Sometimes, one individual in a couple is found to be a carrier of a chromosome translocation. A chromosome translocation is the result of pieces of two chromosomes that have broken off and switched places. All of the chromosome material is still present in an individual with a balanced translocation; it is “balanced” because there is no extra or missing chromosome material. A balanced translocation carrier may have a significant increased risk to have repeated miscarriages or a child with a serious chromosome problem. The goal of preimplantation genetic diagnosis (PGD) for chromosome translocation carriers is to reduce the number of pregnancy losses, and reduce the number of affected offspring.
PGD ProcedureCouples who have PGD will undergo an in vitro fertilization (IVF) cycle to create embryos. Genetic analysis will then be performed on cells from each embryo prior to transfer into the woman’s uterus. To analyze an embryo, we biopsy the embryo around the third day of its development when the embryo has approximately 6-8 cells. One or two cells are taken from the embryo. The embryo is incubated until testing is complete.
The biopsied cells are analyzed using a technique called fluorescence in situ hybridization (FISH). FISH analysis uses DNA probes that can attach to the chromosomes we want to analyze in order to count the chromosomes. Each probe is labeled with a different fluorescent dye (color). The fluorescent probes are applied to the biopsied cell and will attach to specific chromosomes. Under a fluorescent microscope the number of chromosomes of each type (color) can be determined. The FISH analysis involves two rounds of testing for each cell. For couples undergoing IVF and PGD for translocations, the embryos will first be tested for unbalanced translocations. If technically possible, we will also perform a second test for chromosomes 13, 18, 21, X, and Y which are those chromosomes most likely to result in a liveborn child with a chromosome abnormality.
Using FISH analysis we can identify appropriate embryos for transfer. We will not transfer embryos that are found to have an unbalanced chromosome abnormality or aneuploidy. We will not reveal the gender of any embryos.
Preventive Measures During PGD CycleIt is important for a couple undergoing PGD to alter their sexual activity during the PGD cycle. Beginning with day five of treatment with follicle stimulating hormones (FSH), the couple must refrain from sexual intercourse until cycle completion (regular menstruation or positive blood pregnancy test). Intercourse during this time could lead to a pregnancy with an embryo that has not been tested.
Risks of PGDThe biopsy technique required to perform PGD has been in use since 1990. From use of the technique since that time, the chance of accidental damage to an embryo during the removal of cells is very low. The risks of biopsy to an embryo include but are not limited to decreasing the embryo’s chance for survival and continued development prior to implantation. Embryo biopsy is not known to lead to an increase in children born with congenital abnormalities, birth defects, mental retardation, or other possible problems with development following natural fertilization. It is important to know that there may be no embryos available for embryo transfer. PGD may reveal that all embryos are chromosomally abnormal. However, a normal pregnancy outcome cannot be guaranteed whether or not there is an embryo biopsy.
Accuracy of PGDOverall, PGD is an accurate process for determining the chromosomal content of a cell from an embryo. There is a small risk of inaccuracy (incorrect identification of the chromosomal make-up) and/or unclear results in the biopsied embryo(s).
Some embryos are a mixture of normal and abnormal cells (mosaicism). If we analyze a cell that has normal chromosomal content, while unanalyzed cells have a chromosome abnormality, we could inaccurately identify that embryo as being chromosomally normal. Therefore, the analyzed cell may not be representative of the embryo as a whole. Due to the possibility of inaccuracy and the possibility of less common chromosome abnormalities for which we do not test, the option of testing early in pregnancy through prenatal diagnosis is available.
Cost of PGDThe PGD costs are in addition to the cost of in vitro fertilization (IVF) and embryo transfer. They include the cost of the DNA probes, FISH analysis, the biopsy procedure and professional physician and staff fees. There is a non-refundable fee for initial blood chromosome studies for both partners. Once a cycle is begun, there is an additional fee for PGD for translocations for each cycle. Insurance companies generally do not cover the cost of PGD. We will be happy to provide you with a detailed cost analysis for PGD.
Contact InformationRequests for information regarding the Children’s Hospital of Wisconsin, Froedtert Hospital and Medical College of Wisconsin PGD and IVF program can be obtained by contacting the Reproductive Medicine Center at 262-253-9220 or by using our Contact Us form. Information regarding preimplantation genetic diagnosis can be discussed in more detail with a genetic counselor.
Date: June 29, 2005
Last Review Date: March 8, 2013
Online Editor(s): Shannon Krause