Preimplantation Genetic Screening

What is Preimplantation Genetic Screening?

Mount Sinai Services is the first lab in Canada to offer aCGH Preimplantation Genetic Screening (PGS), which screens eggs or embryos for chromosomal abnormalities in all 24 chromosomes using an advanced microarray technology. This procedure enables the selection of only chromosomally normal eggs or embryos for transfer in an IVF/ICSI cycle. Many researchers believe that PGS may improve the chances of pregnancy and delivery.

Why are chromosomes important in fertility?

Approximately 70% of embryos produced either through natural conception or IVF are lost before birth, and most before implantation. A major cause of embryo loss, including miscarriage, is a chromosomal anomaly known as aneuploidy, where there is either a loss or a gain of one or multiple chromosomes. Some of these anomalies are compatible with full-term delivery, such as when there are three copies of chromosome 21 (known as Down Syndrome). Many other anomalies are not compatible with full-term delivery, and result in failed cycles or miscarriages.

The incidence of chromosomal aneuploidies increases with a woman's age. For example, the incidence of Down Syndrome rises from 1 in 900 at age 30, through 1 in 230 at age 37, to 1 in 20 at 46.

This graph shows that as maternal age increases, so does the risk of chromosome anomalies and related miscarriages, resulting in a major reduction in live birth.

Why is aCGH PGS a better approach?

Embryologists cannot distinguish between chromosomally normal and chromosomally abnormal eggs or embryos by simply looking at them through a microscope.

Other PGS techniques use fluorescent DNA probes to allow inspection of specific parts of chromosomes. This method still relies on visual examination under a microscope, and can only identify abnormalities in a limited number of chromosomes .

In contrast, aCGH PGS identifies abnormalities by making thousands of independent measurements of the chromosomes at a molecular level. The data that is obtained results in a computer-generated chromosome map for each embryo sample. The figure above shows PGS results for a 'chaotic' embryo, which has duplications and deletions in multiple chromosome positions.

Mount Sinai's trained scientists interpret the PGS data and provide a detailed genetic report to the IVF physicians in time for embryo transfer.

Using this PGS technology can help identify chromosomally normal eggs or embryos, which it is hoped can lead to:

Improved IVF success rates
Decreases in the number of failed cycles and miscarriages
Decreases in the incidence of multiple pregnancies (by transferring fewer, chromosomally normal embryos)

The impact of this technology on live birth rate is the subject of a number of ongoing studies.

Who is suitable for PGS?

PGS is suitable for all patients, particularly for those patients at higher risk. For example:

Women who are 35 or over
Couples where the men have been shown to have sperm at risk of carrying abnormal chromosomes
Couples who have had several miscarriages
Couples who have had several failed IVF cycles

However, even in younger couples, up to half of a woman’s eggs may carry chromosomal abnormalities. PGS can effectively identify chromosomally normal embryos, which may help to increase the chance of IVF success.

It is very important that each couple has their individual circumstances assessed by their practitioner.

How do we use PGS technology?

PGS requires the biopsy (removal) of cells during one of the following stages:

a. Pre-Fertilization Stage: A tiny cell, called the Polar Body, is removed from the egg. This cell is not used in the fertilization process, but still contains most of the chromosomal information.

b. Blastomere Stage (Day 3 after fertilization): A single cell is removed from the embryo.

c. Blastocyst Stage (Day 5 after fertilization): Several cells are removed from an area of the embryo which becomes the placenta (trophoblast cells), thereby avoiding the cells destined to become the fetus.

Which stage a physician will use depends on many factors.

The biopsied cells are sent to Mount Sinai Hospital where they are analyzed using aCGH array technology. Results are provided to the practitioner in 1-2 days.

Successes using aCGH PGS

More than 50,000 IVF cycles using PGS have been performed world-wide since the method was first reported.

In 2008, a 41-year-old British woman who had been through 13 previous failed cycles of IVF, successfully conceived after aCGH PGS was used to select chromosomally normal embryos for implantation. Baby Oliver was born in 2009, and this event was published in the scientific literature. [PubMed]

Since then, the technology has continued to develop, with the most appropriate strategies designed for each individual couple.

A recent study of 150 cases of IVF carried out in the UK reported tripled rates of embryo implantation following aCGH PGS screening. [J Fertil in vitro 2011 1:1]

Are you interested in learning more about PGS?

Please feel free to contact us at Mount Sinai Services:

E-Mail: mss@mountsinaiservices.com

Phone: (416) 586-4453