Chromosomal aneuploidy is a type of chromosome abnormality that occurs when a cell has either too many or too few chromosomes. The most common example of a chromosomal aneuploidy is Down syndrome, or trisomy 21, which involves an extra chromosome 21. Other common chromosomal aneuploidies are trisomy 13, trisomy 18, Turner syndrome and Klinefelter syndrome, which can result in liveborn children with a range of birth defects and cognitive disability. An extra chromosome 15, 16, or 22, results in a miscarriage early in pregnancy.
The goal of preimplantation genetic diagnosis for chromosomal aneuploidy is to select for and maximize the chance of transferring only chromosomally normal embryos to achieve more pregnancies, reduce the number of miscarriages and reduce the number of affected children.
Risk of Chromosomal Abnormalities
As a woman ages, the chance of having an aneuploid pregnancy increases. There is a significant rate of chromosome abnormalities in embryos, and the frequency of these abnormalities increases with maternal age. Only a few pregnancies carrying an extra or missing chromosome will go to term; most will be miscarried.
|Maternal Age at Delivery||Risk of Aneuploidy in Liveborn Child|
|30||1 in 385 (or 0.26%)|
|35||1 in 192 (or 0.52%)|
|38||1 in 102 (or 0.98%)|
|40||1 in 66 (or 1.5%)|
|45||1 in 21 (or 4.8%)|
|49||1 in 8 (or 12.5%)|
PGD for Sex-Linked Disorders
Most couples at risk for a sex-linked condition are identified by review of the family history or the birth of an affected child. The majority of sex-linked conditions are caused by a change, or mutation, in a gene on the X chromosome and typically affect only males.
Since a male has only one X chromosome, if it has a mutated gene he will develop the disorder. It is this type of inheritance that causes disorders such as Duchenne Muscular Dystrophy, hemophilia and some types of hydrocephalus (water on the brain).
Preimplantation genetic diagnosis can be used to test the sex of embryos so only female embryos, which are not at risk for a sex-linked disorder, can be transferred. A technology called Microsort® can significantly increase the chance of conceiving a female child by selecting for sperm that have an X chromosome.
PGD Procedure for Chromosomal Aneuploidy
Couples electing preimplantation genetic diagnosis undergo an in vitro fertilization (IVF) cycle to create embryos. Genetic analysis is then performed on cells from each embryo prior to transfer into the woman’s uterus. The embryo is biopsied on 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 cells are then carefully analyzed to discover specific genetic problems, such as Down syndrome. Using the latest technology from Gene Security Network, all 24 chromosomes are analyzed, rather than just a few. Embryos found to have a chromosome abnormality are not transferred. The gender of the embryos is not revealed, except for couples who are screening for an X-linked condition.