Embryo screening, scientifically known as Preimplantation Genetic Testing (PGT), is a process performed on embryos created through In Vitro Fertilization (IVF). PGT allows specialists to assess the genetic makeup of an embryo before it is transferred into the uterus. Its primary purpose is to identify embryos with specific chromosomal abnormalities or genetic disorders that can lead to implantation failure, miscarriage, or the birth of a child with a known condition. By providing this health information early, PGT aims to improve IVF success rates and reduce the risk of passing on inherited diseases.
Classification of Screening Tests
Embryo screening is divided into three main categories, each designed to detect a different type of genetic issue.
Preimplantation Genetic Testing for Aneuploidy (PGT-A)
This is the most common form, focusing on the total number of chromosomes within an embryo. A human embryo should have 46 chromosomes. PGT-A screens for missing or extra whole chromosomes, a state known as aneuploidy. This imbalance can cause conditions like Down Syndrome or lead to failed implantation or early pregnancy loss.
Preimplantation Genetic Testing for Monogenic/Single Gene Disorders (PGT-M)
PGT-M is used when parents have a known risk of passing on a specific inherited disease. It tests for mutations in a single gene that cause serious illnesses, such as Cystic Fibrosis, Huntington’s Disease, or Sickle Cell Anemia. This testing requires a custom-designed probe based on the family’s unique genetic mutation before the IVF cycle begins.
Preimplantation Genetic Testing for Structural Rearrangements (PGT-SR)
PGT-SR is reserved for couples where one partner carries a chromosomal rearrangement, such as a translocation or inversion. PGT-SR determines if the embryo has inherited an unbalanced form of this rearrangement, which often results in recurrent miscarriage or birth defects.
The Procedural Steps of Screening
The process of embryo screening begins after fertilization, once the embryos have developed in the laboratory. Screening is typically performed when the embryo reaches the blastocyst stage, about five to six days after fertilization. At this stage, the embryo has differentiated into the inner cell mass (which becomes the fetus) and the trophectoderm (which forms the placenta).
Embryologists perform a trophectoderm biopsy, carefully removing a small cluster of three to ten cells from the outer layer of the blastocyst. This ensures the inner cell mass remains untouched. Once removed, the cells are sent to a specialized genetics laboratory for analysis.
While awaiting the genetic results, the biopsied embryos are immediately cryopreserved, or frozen, using a rapid cooling technique called vitrification. This freezing halts the embryo’s development. The genetic analysis often takes one to two weeks before the results are returned to the clinic.
Interpreting Results and Clinical Implications
The results of PGT-A typically fall into three main classifications that guide clinical decisions. The most favorable result is “euploid,” meaning the tested cells showed the correct number of chromosomes. Euploid embryos have the highest probability of successful implantation and a healthy ongoing pregnancy.
Conversely, an “aneuploid” result indicates an abnormal number of chromosomes, such as a trisomy or a monosomy. Aneuploid embryos are generally not transferred due to a high likelihood of implantation failure, miscarriage, or severe genetic syndrome.
A more complex result is “mosaic,” meaning the biopsied cell sample contained a mixture of both euploid (normal) and aneuploid (abnormal) cells. Mosaicism introduces uncertainty, as the embryo may potentially self-correct, and some mosaic embryos have resulted in healthy live births. The final decision regarding which embryo to transfer is made by the patient and their physician, informed by the PGT results. The goal is always to select the most chromosomally normal and viable embryo to maximize the chance of a successful single-embryo transfer.
The Current Discussion Surrounding Embryo Selection
Embryo screening technology has generated a broad societal conversation regarding its application and accessibility. The primary debate centers on the line between screening for disease prevention and selection for non-medical traits. While PGT-M is widely accepted for preventing serious single-gene disorders, the prospect of using advanced genetic analysis to select for complex traits—known as polygenic screening—raises concerns about a potential shift toward “designer babies.”
Ethical discussions also focus on the implication of discarding aneuploid or mosaic embryos, especially since some mosaic embryos could potentially lead to a healthy birth. Furthermore, the high cost of PGT limits its availability, creating concerns about equitable access to the technology. This conversation highlights the challenge of balancing reproductive autonomy with broader societal values regarding disability and genetic selection.