Preimplantation Genetic Testing (PGT) represents an advanced genetic screening method utilized in conjunction with in vitro fertilization (IVF). It involves analyzing embryos prior to transfer into the uterus. The aim of PGT is to reduce the likelihood of implanting embryos with specific genetic conditions or chromosomal abnormalities. This article focuses on evaluating the accuracy of PGT specifically in determining an embryo’s gender.
PGT and Sex Chromosome Identification
PGT examines cells from an embryo, typically at the blastocyst stage (five to six days after fertilization). These cells are extracted from the trophectoderm, the outer layer that forms the placenta, ensuring the inner cell mass (which develops into the fetus) remains undisturbed. The laboratory then analyzes the genetic material from these biopsied cells.
The analysis identifies all chromosomes, including the sex chromosomes. Humans have 46 chromosomes, arranged in 23 pairs, with one pair determining biological sex. An embryo with two X chromosomes (XX) is identified as female, while an embryo with one X and one Y chromosome (XY) is identified as male. This direct examination of the sex chromosomes allows for the determination of gender as part of the broader genetic screening process.
Documented Accuracy Rates
PGT-A, which includes sex chromosome analysis, is highly accurate for determining an embryo’s gender. Sources indicate an accuracy rate of over 99%, often described as “near 100% accuracy” for gender determination. This high level of precision stems from the direct analysis of the embryo’s chromosomal makeup.
Despite its accuracy, PGT is a screening test and not entirely infallible. While rare, there is a small potential for false positives or false negatives. For added assurance, some parents may choose to confirm PGT results through diagnostic prenatal tests, such as chorionic villus sampling (CVS) or amniocentesis, once a pregnancy is established.
Variables Affecting Results
While PGT offers high accuracy, certain factors can, in rare instances, influence its results or lead to an indeterminate gender outcome. One factor is mosaicism, a condition where an embryo contains a mixture of cells with different genetic compositions. For example, some cells might be chromosomally normal while others carry an abnormality, or different cells might have varying percentages of aneuploid cells. Since only a small sample of cells is biopsied, this sample might not fully represent the entire embryo’s genetic makeup if mosaicism is present. This can lead to a result that does not accurately reflect the whole embryo, resulting in a false positive or false negative.
Technical limitations in the laboratory can also play a role. Obtaining sufficient high-quality genetic material from the embryo biopsy is important for accurate analysis. Although the risk is small, there is a possibility of damaging the embryo during the biopsy procedure. The expertise of the embryologist performing the biopsy can impact the quality of the sample collected, which affects the reliability of the test results. The processes of freezing and thawing embryos, often necessary for PGT, can also affect embryo viability.
Broader Applications of PGT
Beyond gender determination, PGT serves broader medical purposes. Its primary use is screening embryos for chromosomal abnormalities (aneuploidy), an incorrect number of chromosomes. Such abnormalities are a common reason for failed implantation, miscarriage, or birth defects. PGT-A helps identify embryos with the correct number of chromosomes, potentially increasing the chances of a successful pregnancy.
PGT also includes specific tests for inherited genetic conditions. PGT-M focuses on monogenic disorders caused by mutations in a single gene, such as cystic fibrosis or sickle cell anemia. PGT-SR detects structural chromosomal rearrangements, like translocations, that parents might carry and pass to their offspring in an unbalanced form. In these contexts, gender identification is a secondary outcome, derived from the comprehensive genetic analysis to select the healthiest embryos.