Genetic testing can be performed on unfertilized eggs, or oocytes, within assisted reproductive technology (ART). This procedure allows specialists to assess the genetic health of the female reproductive cell before it becomes an embryo. The primary motivation is to identify potential genetic abnormalities originating from the mother. Screening the egg before fertilization helps medical teams make informed decisions about which oocytes should be used for in vitro fertilization (IVF), improving the chances of a successful pregnancy.
The Process of Testing the Egg
The methodology for testing the unfertilized egg is known as Polar Body Biopsy (PBB). This technique analyzes the polar bodies, which are small cells extruded by the oocyte during meiosis, the cell division that reduces the chromosome number. A polar body is a packet of discarded maternal chromosomes not needed for embryonic development.
The first polar body (PB1) is released before fertilization, and the second polar body (PB2) is released shortly after fertilization. Sampling the polar body is less invasive to the egg because the polar body has no function in future embryonic development. The procedure involves carefully creating a small opening in the egg’s outer shell, the zona pellucida, and using a specialized micropipette to remove the polar body.
Once removed, the polar body’s genetic material is analyzed to infer the chromosomal status of the oocyte. The genetic content of the polar body acts as a mirror image of the material remaining in the egg. This analysis is performed using advanced molecular techniques to count or examine the chromosomes. The analysis must be completed quickly to allow for potential fresh embryo transfer, as polar bodies may undergo rapid fragmentation.
What Genetic Issues Can Be Detected
Oocyte testing through Polar Body Biopsy (PBB) primarily detects genetic issues originating from the mother’s egg cell. The most common target is aneuploidy, which refers to an incorrect number of chromosomes. Aneuploidy, such as having an extra or missing chromosome, is a major cause of implantation failure and early miscarriage, and its risk increases significantly with maternal age.
PBB can also be used for Preimplantation Genetic Testing for Monogenic Disorders (PGT-M). This is performed when the woman is a known carrier of a specific single gene disorder, such as cystic fibrosis or spinal muscular atrophy. Analyzing the polar body determines if the egg carries the normal or mutated copy of the gene.
Since the polar body contains only maternal chromosomes, PBB exclusively provides information about the egg’s genetic contribution. It can only detect chromosomal abnormalities or inherited gene mutations passed down from the mother. It cannot screen for genetic errors contributed by the sperm or those that arise after fertilization.
Why Testing the Egg Differs from Testing the Embryo
Testing the unfertilized egg is fundamentally different from testing the embryo, which is more common in IVF. The key distinction is the scope of genetic information obtained. Oocyte testing (PBB) only assesses the maternal genetic contribution, providing a partial picture of the embryo’s health.
Conversely, Preimplantation Genetic Testing of the Embryo (PGT) involves a biopsy of the developing embryo, typically at the blastocyst stage (day five or six). The biopsy removes cells from the trophectoderm, the layer that forms the placenta. This method analyzes both maternal and paternal contributions, providing a more comprehensive genetic assessment.
PBB’s limitations are clear when considering paternal genetics or post-fertilization errors. PBB cannot detect chromosomal abnormalities inherited from the sperm or errors arising during the early stages of cell division in the embryo. For example, PBB would miss a trisomy inherited from the father. Embryo testing detects these paternal and post-fertilization errors, making it a more robust diagnostic tool for overall embryo health. However, PBB allows for the selection of genetically normal eggs before fertilization, avoiding the creation of non-viable embryos. The choice between the two methods often depends on the specific genetic risk factors of the parents.
Weighing the Use of Oocyte Genetic Testing
The decision to use oocyte genetic testing, or Polar Body Biopsy (PBB), involves weighing its unique benefits against its inherent limitations.
Advantages of PBB
Ethical Considerations: PBB allows screening before embryo formation, addressing ethical or religious objections in regions where testing or discarding embryos is restricted.
Safety: PBB is considered less physically harmful to the future embryo, as the polar body is a non-essential cell.
Timing: The early diagnosis allows for faster results, potentially enabling a fresh embryo transfer in the same IVF cycle, rather than waiting for analysis of a frozen embryo.
Initial Screening: For women with a high probability of aneuploidy due to advanced age, PBB acts as an initial screening step to avoid fertilizing eggs that are highly likely to be abnormal.
Limitations of PBB
The major drawback is that PBB only screens for maternal errors. This means a genetically “normal” egg could still result in an abnormal embryo due to paternal factors or new errors. This limitation means PBB is often viewed as a screening test rather than a definitive diagnosis. Furthermore, the complexity of chromosome segregation in the egg can sometimes lead to misdiagnosis, requiring that any pregnancy achieved still undergo standard prenatal testing for confirmation.