Preimplantation Genetic Screening
What is PGS?
“Preimplantation genetic screening” (PGS) is used when the parents have no known mutations that are expected to cause disease in the embryo, and are presumed to have a normal karyotype. PGS tests the embryo to confirm that the embryo has an even number of chromosomes.
PGS involves a standard biopsy procedure performed on embryos in conjunction with an in-vitro fertilization (IVF) cycle, prior to implantation. This allows screening of the embryos for chromosomal abnormalities before being transferred in an IVF cycle.
Chromosomal abnormalities can lead to failed implantation or early loss of pregnancy. Screening enables specialists to identify embryos with normal structure and having the best chance for successful implantation.
PGS with Chromosomal Microarray Analysis (CMA)
This new method of testing is more thorough than the past techniques in allowing the identification of healthy embryos because PGS with Chromosomal Microarray Analysis (CMA) requires a biopsy of a few cells from a day 5 or 6 embryo; thereby allowing the lab to screen all 23 pairs of chromosomes, and a pair of sex chromosomes.
New 24 Chromosome Analysis:
Once the biopsy is performed, the genetic material is extracted from the cells. A laboratory specializing in genetics will examine the material to determine that the normal chromosomal compliment is present
FIG. 1 & 2 EMBRYO BIOPSY
The genetic material is first amplified using polymerase chain reaction (PCR) to provide ample material for analysis.
Currently, this genetic testing is more commonly used to determine chromosome copy number. A normal number is 23 pairs of chromosomes. This is also called, by some, “24-chromosome analysis” as there are 22 autosomes, and the sex chromosomes, X and Y, that are analyzed. Once the genetic material from the biopsy is amplified, the product is examined with one of several techniques including: Comparative Genomic Hybridization (CGH), Array CGH, Single-Nucleotide Polymorphism (SNP) Microarray, and Next Generation Sequencing. All of these techniques for PGS are available at Houston Fertility Institute.
Identification and transfer of normal embryos leads to increased pregnancy and delivery rates with decreased miscarriages due to an abnormal number of chromosomes (e.g. trisomy 21, or Down Syndrome).
Who will benefit from preimplantation genetic screening?
PGS may be recommended for:
- Patients who have experienced recurrent miscarriages
- Multiple IVF failures
- Advanced maternal age (>35)
- Desire to limit number of embryos transferred
- Couples with severe male factor infertility
Any patient undergoing IVF can benefit from preimplantation genetic screening.