Preimplantation Genetic Screening (PGS) is a laboratory test performed on embryos during in vitro fertilization (IVF) treatment. It examines the chromosomes within an embryo to determine if the correct number is present. This screening identifies embryos with chromosomal abnormalities before transfer, helping select those with a higher likelihood of leading to a successful pregnancy.
The Purpose of Preimplantation Genetic Screening
PGS is performed to identify chromosomal abnormalities in IVF embryos. Its main objective is to enhance the chances of a successful pregnancy and reduce miscarriage risk. Chromosomal imbalances are a significant factor contributing to implantation failure and early pregnancy loss. By screening embryos, fertility specialists can prioritize those with a normal chromosomal makeup for transfer, improving IVF outcomes.
This genetic screening guides embryo selection, helping avoid the transfer of embryos unlikely to implant or develop normally. PGS aims to optimize IVF efficiency by increasing implantation rates.
The Preimplantation Genetic Screening Process
The PGS process begins after eggs are fertilized in the IVF laboratory and embryos develop for several days. An embryo biopsy is typically performed when the embryo reaches the blastocyst stage, around day five or six. At this stage, a few cells are carefully removed from the trophectoderm, the outer layer that will eventually form the placenta.
The biopsied cells are sent to a specialized genetic laboratory for analysis. Modern techniques like Next-Generation Sequencing (NGS) are used to examine all 23 pairs of chromosomes for numerical abnormalities. This analysis allows for a comprehensive assessment of the embryo’s chromosomal status. Following analysis, chromosomally normal embryos are selected for transfer to the uterus, while others can be cryopreserved.
Chromosomal Abnormalities Identified by PGS
PGS screens for aneuploidies, conditions where an embryo has an abnormal number of chromosomes. A typical human cell contains 46 chromosomes, arranged in 23 pairs. Aneuploidy occurs when there is an extra or missing chromosome in one or more of these pairs.
Examples of aneuploidies PGS identifies include trisomies (an extra copy of a chromosome), such as Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), and Trisomy 13 (Patau syndrome). The screening also detects monosomies (absence of a chromosome) and sex chromosome aneuploidies like Turner syndrome (XO) and Klinefelter syndrome (XXY). This screening is distinct from testing for single gene disorders.
Who May Consider Preimplantation Genetic Screening
Individuals or couples undergoing IVF may consider PGS when factors increase the likelihood of chromosomal abnormalities in embryos. Advanced maternal age (over 35 years) is a common indication, as aneuploidy incidence in eggs increases with age. Couples with recurrent pregnancy loss may also find PGS beneficial, as chromosomal imbalances are a frequent cause of miscarriage.
PGS can also be considered after multiple failed IVF cycles. Identifying chromosomally normal embryos can help improve the success rate in subsequent attempts. Couples with a history of previous pregnancies affected by chromosomal abnormalities might choose PGS to reduce the risk of similar outcomes.
Understanding Preimplantation Genetic Screening Results
PGS results categorize embryos primarily as euploid, aneuploid, or mosaic. A euploid result indicates the embryo has the correct number of chromosomes, suggesting a higher chance of successful implantation and a healthy pregnancy. Conversely, an aneuploid result means the embryo has an abnormal number of chromosomes; such embryos are typically not transferred due to their low viability and increased risk of miscarriage or genetic conditions.
Some embryos may be classified as mosaic, meaning they contain a mixture of chromosomally normal and abnormal cells. While PGS is a highly accurate screening tool, it is a screening test, not a diagnostic one. Therefore, further prenatal testing during pregnancy might be recommended to confirm the chromosomal status of the developing fetus.