What does Chromosome Screening Entail?


Chromosome Screening can be used in conjunction with conventional IVF treatment to increase the chances of the female giving birth to a healthy child. During the IVF treatment, eggs are harvested from the female and fertilised. Some embryos are then usually returned to the female’s uterus in the hope that the normal pregnancy process can begin.

With chromosome screening, the selected number embryos are sent for analysis to ensure that they have the right number of chromosomes. The purpose of this is to reduce the chances of the foetus having congenital disabilities such as Down Syndrome. The process is conducted in a lab before the embryos are returned to the female.

What Happens in Chromosome Screening?

After the eggs have been fertilised using in vitro fertilisation (IVF), a small number of cells will be taken from each embryo and sent for a biopsy in a genetics laboratory. The process, called aneuploidy, is the selecting of only healthy embryos. Embryos that have too few or too many chromosomes are the reason for 60% of miscarriages and are the main reason why pregnancy doesn’t occur during an IVF cycle.

Preimplantation Genetic Screening (PGS)

PGS is often referred to as aneuploidy screening and encompasses Next Generation Sequencing (NGS) and is a proven method of selecting healthy embryos that do not have any chromosome abnormalities. It is a method that significantly increases the chances of pregnancy as implantation rates are higher with the possibilities of miscarriage markedly improved.

Which Women are Suitable for Comprehensive Chromosome Screening?

  • Those aged 35 or over
  • Those whoever have a history of miscarriage
  • Those with a history of chromosome abnormalities
  • Those who have repeatedly had failed IVF cycles

What are the Main Causes of Abnormal Chromosomes?

It is actually something that is quite common and happens during the regular production of eggs and sperm during the early stages of embryo development. At this stage, the cells, which comprise of eggs, sperm and embryo divide. The division can result in too few or too many chromosomes; this will resulting in missing or added DNA in each embryo.

Is it Safe for the Embryos?

There is always a small element of risk, as the process involves removing several cells from an embryo. It may cause some damage which may prevent the embryo from developing in the womb.

Research has also shown that in some cases, the cells in the embryo are not identical. It is called mosaicism. In order for the results to be conclusive, a more significant number of cells may need to be taken to give a more representative result. Mosaicism can lead to some misleading results returning from laboratory testing. A professional clinic offering Chromosome Screening will take all possible steps to prevent this from happening.

Are There Any Other Potential Risks?

The risks involved with PGS are very similar to those associated with regular IVF treatment. However, it should be recognised that PGS does carry some additional risks such as misdiagnosis and damage caused to the embryo when the cells were removed. The risks increase as a woman gets older.

Are There Any Other Techniques Available?

Fluorescent in situ hybridisation (FISH)

FISH is used to establish where a particular gene falls in a person’s chromosome. The process involves preparing a short sequence of single-stranded DNA which is an exact match to that specific gene. The single-stranded DNA is known as a probe. The probes are then labelled using fluorescent dye. Regular DNA is composed of two strands of complementary molecules which bind to each other. The complementary strands will link up to each other, but the fluorescent will make it easy for the researchers to be able to establish the location.

When is FISH used?

Researchers use three different types of FISH probes. Each of the probes will have a different purpose. These are:

  • Locus specific probes: As these types of probes bind to a particular region of a chromosome, they help researchers to a proportion of the gene or to establish where it is located and if several copies exist.
  • Alphoid or centromeric probes: These are repetitive sequences that can be found in the centre of each chromosome. These are primarily used to establish if the correct number of chromosomes are present and if a person has missing or additional genetic material.
  • Whole chromosome probes: These are a collection of probes which bind together. Researchers use this method of examining chromosomal abnormalities. The process is now used less frequently with many of its applications being replaced by microarrays. However, it still commonly used to study comparisons between chromosomal arrangements in genes of inter-related species.

Next-Generation Sequencing (NGS)

NGS is the latest method of chromosome screening and involves the genetic testing of all 23 pairs of chromosomes. The screening is conducted at a far more advanced level using high-resolution equipment. It is frequently used to screen embryos to establish any issues such as Down Syndrome or other similar aneuploidy conditions. It is, therefore, an essential tool when it comes to chromosome screening.

What is Involved in NGS?

NGS is very similar to PGS in that involves the removal of several cells from the embryo and conducting chromosomal tests in a laboratory. Using revolutionary equipment such as a microarray or DNA chip, NGS amplifies the DNA in the cell’s nucleus is compared with regular male and female DNA to establish if there are any abnormalities.

If any chromosomal abnormalities are detected, they will be removed from possible embryo transfer. Only embryos that display healthy DNA will be transferred back to the female, which will significantly increase her chances of becoming pregnant.

Most biopsies are performed on Day 5 embryos although advanced clinics will have the capacity to perform biopsies throughout the development process depending on the patient’s individual circumstances.