Embryo Genetic Screening

Aneuploidy |  Preimplantation Genetic Diagnosis (PGD) and Preimplantation Genetic Screening (PGS)  |  The process of embryo screening

Aneuploidy

Most cells in the human body have 23 pairs of chromosomes – one copy of each pair is inherited from the mother and the other copy is inherited from the father. The first 22 pairs of chromosomes (referred to as autosomes) are numbered from 1 to 22, and are arranged from largest to smallest. The 23rd pair of chromosomes are the sex chromosomes. Females normally have two X chromosomes (XX), while males normally have one X chromosome and one Y chromosome (XY). Aneuploidy occurs when the cells that make up our body divide but the chromosomes do not separate evenly between the two cells. This means the cells have an abnormal number of chromosomes (either too many or too few), which in the embryo can result in genetic disorders in children or result in miscarriage during pregnancy. If there is an error in division of the egg’s chromosomes and one of the pairs of chromosomes fails to separate the egg or embryo will have an uneven number of chromosomes. In Down’s syndrome, for example, the embryo has a total of forty-seven chromosomes instead of forty-six because it has three copies of chromosome 21 instead of the normal two copies. This kind of chromosome error, in which one of the chromosomes has three copies instead of the proper two copies, is called “trisomy.”

All of these chromosomal errors, including trisomies, monosomies, and various combinations can occur in virtually any of the chromosomes, and these errors, as a group, are called “aneuploidy.” These numerical chromosomal errors in a cell can be diagnosed by embryo screening. The aging process of eggs makes it harder for them to undergo normal chromosome separation. This leads to an increase in genetic abnormalities in resultant embryos, even though these embryos may appear normal on visual assessment. Studies have shown that rates of aneuploidy are increased in older women.

Preimplantation Genetic Diagnosis (PGD) and Preimplantation Genetic Screening (PGS)

PGD and PGS are specialised laboratory techniques that provide diagnostic information concerning an embryo prior to its transfer to the uterus. PGD allows us to assess IVF embryos for chromosomal abnormalities, such as missing or extra chromosomes. Traditional screening allows the embryo to be screened for a specific disorder so that only embryos free of the disorder are considered for transfer to the uterus for a chance to establish pregnancy. New screening methods allow us to look at all chromosomes present in the embryo, meaning we can make a more informed decision about which embryo to transfer. Couples who have a serious inherited genetic condition, or a family history of such, or who are at a higher risk of having chromosomal abnormalities in their embryos may consider screening their embryos, for example:

• Patients who are at risk of having a child with an inherited genetic condition eg: haemophilia, thalassaemia, cystic fibrosis, Huntington’s disease, Duchenne Muscular Dystrophy
• Sex linked disorders
• Known history of balanced translocation (a chromosome rearrangement) in one or other of the couple
• Previous chromosomal abnormality in a pregnancy
• Recurrent miscarriage (repeated early pregnancy loss)
• Implantation failure (repeated embryo transfers without pregnancy)

The process of embryo screening