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By screening for commonly encountered embryonic chromosomal disorders, infertile couple may benefit from this test if the female partner is over the age of 35 or if they have had several failed IVF cycles. In addition, couples suffering from recurrent pregnancy loss may benefit from PGD.
Chromosomes are sub-cellular structures that house one’s genes. All of the information that is needed to lead to an individual’s development is contained on these chromosomes and each one of us has 23 pairs of them. During human reproduction, it is not unusual to produce embryos that have too few or too many chromosomes. This is known as aneuploidy (too few or too many chromosomes). While most pregnancies involving an extra chromosome 13, 18, 21, X or Y chromosome will end in miscarriage, these chromosomal abnormalities are observed in some live born. Down syndrome is a form of aneuploidy in which the individual has 3 copies instead of 2 copies of chromosome 21. Live born with abnormalities in chromosome 13 and 18 manifest more severe findings than Down syndrome, while those involving X and Y are less severe.
As a women ages, she is more apt to mature eggs that contain an abnormal number of chromosomes. The risk of having a liveborn with a chromosomal abnormality increases with age: 1/385 at age 30, 1/192 at age 35 and 1/42 at age 42. Increases in chromosomal abnormalities is observed in some case of recurrent pregnancy loss and has been observed amongst couples experiencing multiple IVF failures. Aneuploid embryos are less likely to implant than normal embryos. When they do implant, aneuploid embryos almost always miscarry. PGD was designed to screen for some of the more common chromosomal abnormalities in order to help select which embryos to transfer to the uterus in order to optimize the selection process.
PGD for aneuploidy screening does not offer information on birth defects and cannot detect a genetic illness. It does offer information on 8 of the most problematic chromosomal abnormalities. By selecting the embryos felt to be normal, implantation rate can be improved and the risk of miscarriage is reduced. Finally, the likelihood of giving birth to a child with a chromosomal abnormality is reduced.
PGD for aneuploidy is also recommended for women of any age that have given birth to a trisomic child. It is suggested for women who have carried a trisomic pregnancy beyond the first trimester.
We must emphasize that all 23 pairs of chromosomes are not screened. Currently, such comprehensive screening is not technically possible. Further, it is possible that the sampled cell has a different chromosomal complement than the rest of the embryo. This is a condition known as mosaicism and it limits the predictive power of the test. This test is conservatively structured so that it reduces the likelihood of transferring an abnormal embryo. This means that there is a false positive reading in roughly 10% of embryos. In addition, it is possible that the procedure can damage an embryo or that an embryo may not be assessed owing to technical issues with the test. Finally, it is possible that none of the embryos screened are normal and therefore no embryo transfer performed.
The risks of PGD remain unclear. the main concern is the microsurgical procedure used to remove the sampled cell from the embryo. While animal and human data suggest that this is safe, the data are limited.
Procedure:
On day 3 post retrieval, a small opening is made in the protein coat surrounding the embryo. A biopsy pipette is introduced into the embryo and a single cell is extracted from the embryo. This cell is then transferred onto a slide and the contents of the cell ruptred to allow the chromosomes to stick to the glass. The chromosomes are then labelled with fluorescent stains to allow for the visualization of the chromosomes. This is a process called florescence in-situ hybridization or FISH. The cells are then analyzed by a geneticist and a reading is given for each successfully sampled embryo regarding the chromosomes for which they are being screened.
In general it takes 24-30 hours to get the results of the test. Most embryo transfers following PGD for aneuploidy screening will take place on post retrieval day 4. On occasion, they may not be done until post retrieval day 5.
If a pregnancy results following PGD, we strongly recommend close monitoring for growth via ultrasound (our routine on all IVF pregnancies) and a chromosomal analysis of the fetus during the second trimester.
PGD does add cost to the cycle and is not covered by most insurances.
