Preimplantation Genetic Screening (PGS)
What is PGS?
Preimplantation genetic screening, known as PGS, is a revolutionary technique that allows examination of embryo chromosomes before they are transferred to the maternal uterus. This tool has become a fundamental ally for couples seeking to increase their chances of having a successful pregnancy and a healthy baby.
The process behind the technique
PGS works through a careful cellular biopsy of the embryo, followed by a detailed genetic analysis. Specialists specifically look for what is known as euploidy, which is simply having the correct number of chromosomes. It’s like verifying that all puzzle pieces are present before starting to assemble it.
During natural fertilization, each embryo receives 23 chromosomes from the father through the sperm and another 23 from the mother through the egg, totaling 46 chromosomes organized in 23 pairs. This is the perfect configuration that nature has designed for normal human development.
When things go wrong: aneuploidy
Unfortunately, things don’t always go according to plan. When an embryo has an abnormal number of chromosomes, a condition known as aneuploidy, significant complications can arise. This situation can manifest as having an extra chromosome (trisomy) or one missing (monosomy), and the consequences are usually pregnancies that don’t reach term or babies with significant birth defects.
The most well-known chromosomal abnormalities include Down syndrome, caused by an extra copy of chromosome 21, Edwards syndrome due to trisomy of chromosome 18, and Turner syndrome, where an X chromosome is missing in females. These conditions illustrate why chromosomal accuracy is so important for normal development.
Who benefits from PGS?
This technique is especially valuable for women of advanced maternal age, typically considered from age 35 onwards, as the risk of chromosomal abnormalities increases significantly with age. It is also an important option for couples who have experienced recurrent miscarriages or repeated implantation failures in previous fertility treatments.
Couples with a history of chromosomal abnormalities or alterations in their own karyotype also find PGS to be an invaluable tool for improving their chances of reproductive success.
The advantages it offers
PGS provides multiple benefits to the assisted reproduction process. Primarily, it dramatically reduces the risk of having a baby with chromosomal abnormalities and improves successful implantation rates. This translates to fewer spontaneous miscarriages and greater confidence in each embryo transfer.
An additional advantage is that it allows specialists to recommend single embryo transfer with greater safety, thus reducing the risks associated with multiple pregnancies while maintaining high success rates.
The complex side of the technique
Like all advanced medical technology, PGS has its limitations. One of the most important is the phenomenon known as embryonic mosaicism, where the same embryo can have cells with different chromosomal compositions. This can complicate result interpretation and occasionally lead to incorrect diagnoses.
Additionally, the biopsy is performed on cells that will eventually form the placenta, not necessarily the baby, which adds another layer of complexity to result interpretation.
The step-by-step procedure
The process begins with a standard in vitro fertilization cycle, where ovaries are stimulated, eggs are extracted and fertilized in the laboratory. Embryos are cultured until they reach the blastocyst stage, approximately five to six days after fertilization.
At this point, a very delicate biopsy of the trophectoderm is performed, the part of the embryo that will become the placenta. The embryo is immediately preserved through vitrification while the extracted cells are sent to the genetics laboratory for analysis.
Genetic analysis uses advanced sequencing technologies to examine all chromosomes. Results are generally available in one to two weeks, classifying each embryo as euploid (chromosomally normal), aneuploid (with abnormalities), or non-informative (when a clear result cannot be obtained).
Interpreting the results
Embryos classified as euploid are considered the best candidates for transfer, as they have the highest probability of resulting in a successful pregnancy and healthy baby. Aneuploid embryos are generally not recommended for transfer due to the high risk of implantation failure or abnormalities.
The final decision on which embryo to transfer also considers other factors such as the morphological quality of the embryo and the specific circumstances of each couple.
Looking to the future
PGS technology continues to evolve rapidly. Researchers are working on less invasive methods, such as analyzing the free DNA that embryos release into their culture media, which could eliminate the need for biopsy in the future.
Artificial intelligence is also beginning to play a role in result interpretation, promising greater precision and efficiency in preimplantation genetic diagnosis.
Final reflections
PGS represents an extraordinary advance in reproductive medicine, offering couples a powerful tool to improve their chances of having a healthy baby. Although the technique has its complexities and limitations, the benefits it provides have transformed the landscape of assisted reproduction.
As with any important medical decision, the choice to use PGS should be individualized for each couple, considering their specific circumstances, medical history, and reproductive goals. Consultation with reproductive medicine specialists can help determine if this technology is the most appropriate option for each particular situation.
