Embryo screening is a set of genetic tests performed on embryos created through IVF before they are transferred to the uterus. Clinicians remove a tiny sample of cells from a developing embryo, analyze those cells for chromosomal or genetic abnormalities, and use the results to help decide which embryo to transfer. The formal name is preimplantation genetic testing, or PGT, and it has been practiced in various forms since the 1990s. Whether it genuinely improves your chances of having a baby, though, turns out to be a more complicated question than many fertility clinics let on.
Three Kinds of Embryo Screening
Not all embryo screening looks for the same thing. There are three distinct types, each targeting a different category of genetic problem.1PubMed Central. Preimplantation genetic testing: A narrative review
- PGT-A (aneuploidy): This screens for embryos with the wrong number of chromosomes. A healthy human cell has 46 chromosomes, and having too many or too few (a condition called aneuploidy) is the most common reason embryos fail to implant or miscarry early. PGT-A is the most widely used type and the one most people mean when they say “embryo screening.”
- PGT-M (monogenic disorders): This targets specific inherited diseases caused by a single gene, such as cystic fibrosis, sickle cell disease, or Huntington’s disease. It is used when one or both parents carry a known genetic mutation and want to avoid passing it to a child.
- PGT-SR (structural rearrangements): This looks for large-scale chromosomal rearrangements like translocations, deletions, or duplications. People who carry balanced translocations often have no symptoms themselves but produce a high proportion of embryos with unbalanced chromosomes, leading to repeated miscarriages.
The goal of PGT-M is to select embryos that either lack the harmful mutation entirely or, in the case of recessive conditions, are healthy carriers.1PubMed Central. Preimplantation genetic testing: A narrative review PGT-SR and PGT-A share a somewhat overlapping logic: both try to identify embryos with the best chromosomal profile for a successful pregnancy. But the clinical situations that call for each are different, and the laboratory protocols vary accordingly.
How Cells Are Collected From the Embryo
The physical step that makes any of this possible is the biopsy. After eggs are fertilized in the lab, embryos are grown for five to six days until they reach the blastocyst stage, a hollow ball of roughly 100 to 200 cells. At that point, a laser is used to open a small hole in the outer shell, and an embryologist carefully removes about five to ten cells from the trophectoderm, the outer layer that will eventually become the placenta rather than the baby itself.
This approach replaced an older technique that removed one or two cells from a three-day-old embryo, which was riskier and less accurate. Because the trophectoderm biopsy takes cells from the placental lineage rather than the inner cell mass (the part that becomes the fetus), it is considered less invasive to the developing embryo. Research on outcomes after trophectoderm biopsy has been broadly reassuring for newborn health: neonatal outcomes from biopsied and unbiopsied embryos appear comparable.2PubMed Central. Trophectoderm biopsy is associated with adverse obstetric outcomes rather than neonatal outcomes That same study did flag a higher risk of gestational hypertension and abnormal umbilical cord in pregnancies from biopsied embryos, though it also found a possible protective effect against premature rupture of membranes. A separate analysis of frozen single embryo transfer cycles concluded that blastocyst biopsy did not increase the risk of pregnancy complications or negatively affect early motor development in infants.3PubMed Central. The obstetrical and infant outcomes of trophectoderm biopsy on preimplantation genetic testing embryos after frozen single embryo transfer cycles
One practical consideration is that biopsied embryos are almost always frozen while waiting for the genetic results to come back, which typically takes one to two weeks. That means the embryo transfer happens in a subsequent cycle rather than the same one. Repeated rounds of freezing and thawing can take a toll: embryos that undergo an additional freeze-thaw cycle before biopsy show lower blastocyst formation rates and lower high-quality blastocyst rates compared to those that do not.4PubMed Central. Impact of repeated cryopreservation on embryo development and chromosomal ploidy
What Happens in the Lab
Once the biopsied cells arrive in the genetics lab, the DNA is amplified and analyzed. The technology has evolved substantially. Early screening in the 1990s relied on a technique called fluorescence in situ hybridization, which could only check a handful of chromosomes at a time. That gave way to microarray methods and eventually to next-generation sequencing platforms, which can assess all 23 chromosome pairs simultaneously.5PubMed Central. Changes in sequencing technology used for preimplantation genetic testing for aneuploidy The shift to comprehensive chromosome screening is one reason modern PGT-A is considered more reliable than its predecessors, though “more reliable” does not mean infallible.
For PGT-M, the lab needs advance preparation. Because it is looking for a specific mutation rather than a broad chromosomal count, the family’s DNA must be studied beforehand to design a customized test. As long as the disease-causing gene has been clearly identified, PGT-M can theoretically be offered for any single-gene disorder.6Human Reproduction. O-114 Preimplantation genetic testing for monogenic diseases without necessary familial members In practice, this setup phase adds weeks and significant cost to the process.
The Mosaicism Problem
Here is where embryo screening gets genuinely messy. A mosaic embryo is one that contains a mixture of chromosomally normal and abnormal cells. Because the biopsy only samples a small number of cells from the outer layer, the result may not reflect what is happening everywhere in the embryo. An embryo called “aneuploid” based on its biopsy could have a perfectly normal inner cell mass, and vice versa.
This is not a rare edge case. Mosaicism is common in early embryos, and the clinical question of what to do with a mosaic result has become one of the most debated topics in reproductive medicine. Data from a large international registry of over 3,000 mosaic embryo transfers showed that the level of mosaicism matters: embryos classified as low-mosaic had a roughly 40% ongoing pregnancy and live birth rate, compared to about 29% for high-mosaic embryos.7Human Reproduction. O-154 Chromosomal, gestational, and neonatal outcomes of mosaic embryos: analysis of 3074 cases from the international registry of mosaic embryo Strikingly, the mosaicism detected at the blastocyst stage was reflected as true fetal mosaicism in prenatal testing in fewer than 1% of those pregnancies.7Human Reproduction. O-154 Chromosomal, gestational, and neonatal outcomes of mosaic embryos: analysis of 3074 cases from the international registry of mosaic embryo
The type of mosaicism also matters. A meta-analysis found that embryos with segmental mosaicism (where only part of a chromosome is affected) had outcomes similar to fully normal embryos, while those with whole-chromosome mosaicism fared worse. Within that category, trisomy mosaics had markedly lower live birth rates than euploid embryos, while monosomy mosaics did not show a significant difference.8PubMed. Impact of different types of embryonic mosaicism on pregnancy outcomes
One explanation for why mosaic embryos sometimes produce healthy babies is that embryos appear capable of a degree of self-correction. Research has found that aneuploid cells can be selectively pushed into the trophectoderm (the future placenta) and depleted from the embryonic germ layers through a process involving programmed cell death.9Nature Cell Biology. Depletion of aneuploid cells in human embryos and gastruloids Several mechanisms for this have been proposed, including the growth advantage of normal cells in a mixed population and the extrusion or correction of extra chromosomes.10Stem Cells and Development. Self-Correction of Chromosomal Abnormalities in Human Preimplantation Embryos and Embryonic Stem Cells An experiment examining blastocysts alongside their expelled cell debris found that some embryos classified as chromosomally normal had shed aneuploid cells during development, suggesting the embryo had effectively cleaned house.11PubMed Central. Do human embryos have the ability of self-correction?
All of this means that PGT-A results are not a binary pass/fail. Embryos labeled as mosaic or even aneuploid may still lead to healthy pregnancies, and some embryos labeled euploid may still fail to implant. The biopsy is a snapshot of a handful of cells at one moment in time, not a complete readout of the embryo’s genetic fate. This uncertainty is a real source of stress for patients, who must weigh the risk of discarding a viable embryo against the risk of transferring one unlikely to succeed.
Does PGT-A Actually Improve Your Chances?
This is the question that divides fertility specialists, and the honest answer is: it depends on how you measure and whom you are testing. One retrospective single-center study found a live birth rate of about 46% in the PGT group versus about 35% in the conventional IVF group per transfer.12PubMed Central. Live birth rates with and without preimplantation genetic testing: a single-center retrospective study That looks like a clear win for screening. But per-transfer rates can be misleading because PGT-A eliminates embryos from the pool before transfer. If you had five embryos and screening deemed three abnormal, you are transferring from a smaller pool. The per-transfer success rate goes up, but your cumulative chance of a baby from the full cycle may not.
A large randomized controlled trial published in the New England Journal of Medicine made this point sharply. It found that live births occurred in about 77% of the PGT-A group compared to about 82% of the conventional IVF group, a difference of nearly five percentage points favoring skipping the screening.13PubMed. Live Birth with or without Preimplantation Genetic Testing for Aneuploidy A UK registry study echoed this finding, reporting that PGT-A was associated with reduced cumulative live birth rates over a full treatment episode.14Reproductive BioMedicine Online. PGT-A is associated with a reduced live birth rate in routine practice: evidence from the UK ART register
How can a test that identifies abnormal embryos lead to fewer babies overall? The mosaicism issue is part of the answer: some embryos flagged as abnormal would have self-corrected and resulted in healthy pregnancies. Another factor is that the biopsy and freezing process itself may harm some embryos. PGT-A also adds cost and time, which can lead patients to abandon treatment sooner. The evidence for screening is stronger in older patients, since the proportion of aneuploid embryos climbs sharply with age. One study found aneuploid embryos made up about 69% of the pool in older patients versus about 40% in younger ones.15PubMed Central. Next-Generation Sequencing (NGS)-Based Preimplantation Genetic Testing for Aneuploidy (PGT-A) of Trophectoderm Biopsy for Recurrent Implantation Failure (RIF) Patients: a Retrospective Study When a large fraction of your embryos are truly abnormal, screening to avoid futile transfers is more likely to be worthwhile. For younger patients with plenty of embryos to work with, the math is less favorable. A cost-effectiveness review noted that age, reproductive timeline, and economic burden all factor into whether PGT-A makes financial sense.16PubMed. A Review of Cost-Effectiveness of Preimplantation Genetic Testing for Aneuploidy
Screening for Single-Gene Conditions
The case for PGT-M is generally more straightforward than for PGT-A. If you know you carry a mutation for a serious genetic disease, selecting an embryo that does not carry that mutation prevents the disease in a way no other intervention can. The technique has been called revolutionary in reproductive medicine for this reason.17PubMed Central. Preimplantation Genetic Testing for Genetic Diseases: Limits and Review of Current Literature
What has changed over the past two decades is who is being referred for PGT-M. It used to be almost exclusively families with a known history of a genetic disease: a sibling with cystic fibrosis, a parent with Huntington’s. Now, expanded carrier screening panels routinely test prospective parents for hundreds of conditions, many of which they have never heard of and have no family history of. This has shifted the referral pattern dramatically. In one long-term tracking of PGT-M for conditions like phenylketonuria (PKU) and hereditary hearing loss, the vast majority of recent cases were prospective, meaning the couples had no affected relatives and were identified only through carrier screening.18Obstetrics Gynecology and Reproductive Sciences. Preimplantation Genetic Testing (PGT) for Non-Lethal Correctable Conditions Through Population Carrier Screening
This raises questions about which conditions are serious enough to warrant the physical, emotional, and financial demands of IVF with PGT-M. PKU, for example, is treatable with a strict diet. Hereditary hearing loss is managed with hearing aids or cochlear implants. These are real conditions with real impacts, but they are not life-threatening. The expanding scope of what PGT-M is used for reflects broader changes in how society thinks about genetic disease and reproductive choice.
False Positives, False Negatives, and the Limits of a Biopsy
PGT is not a perfect diagnostic. False positives (calling an embryo abnormal when it is actually fine) lead to viable embryos being discarded. False negatives (calling an embryo normal when it is not) can result in failed transfers or early pregnancy loss.19PubMed Central. Re-Examination of PGT-A Detected Genetic Pathology in Compartments of Human Blastocysts: A Series of 23 Cases The fundamental limitation is biological: a small biopsy from one part of the embryo cannot perfectly represent the whole. As described earlier, mosaicism means the trophectoderm cells that are sampled may have a different chromosomal makeup than the inner cell mass cells that will become the baby.
For couples going through IVF, the emotional weight of these decisions is substantial. Patients must grapple with moral and ethical questions about the value of embryos, tolerate the physical demands of treatment, and contemplate what to do when test results are uncertain or mosaic. Failed cycles add another layer, forcing the question of whether to try again.20PubMed Central. Patients’ preimplantation genetic testing decision-making experience: an opinion on related psychological frameworks Clinics vary widely in how they counsel patients about mosaic results, and there is no universal consensus on which mosaic embryos should be transferred versus set aside. Compared to euploid embryos, mosaic embryo transfers do result in lower live birth rates overall, but the gap is modest for certain types of mosaicism.21PubMed Central. Rates of live birth after mosaic embryo transfer compared with euploid embryo transfer
Non-Invasive Testing and Polygenic Screening
Because the biopsy is the most physically intrusive step and a source of diagnostic uncertainty, researchers have been working on non-invasive alternatives. The most promising approach analyzes cell-free DNA that embryos naturally shed into the culture medium they grow in. Instead of removing cells from the embryo, the lab simply collects the spent medium and sequences the DNA fragments floating in it.
Results so far are mixed. One study found an overall concordance rate of about 82% between non-invasive testing and standard trophectoderm biopsy, with aneuploidy agreement at roughly 92% but euploidy agreement lower at about 76%.22PubMed Central. Embryonic Cell-free DNA in Spent Culture Medium: A Non-invasive Tool for Aneuploidy Screening of the Corresponding Embryos Another study found that when both methods were compared against the actual chromosomal status of extended embryo outgrowths, non-invasive testing performed at least as well as standard biopsy, with neither method showing high concordance rates overall.23PLoS ONE. Cell-free DNA in spent culture medium effectively reflects the chromosomal status of embryos following culturing beyond implantation compared to trophectoderm biopsy The technology is not yet ready to replace biopsy-based testing, but it represents a plausible future direction.
A more controversial frontier is polygenic embryo screening, which attempts to rank embryos based on risk scores for complex conditions like heart disease, diabetes, or schizophrenia. Unlike single-gene disorders where one mutation drives the disease, these conditions are influenced by thousands of genetic variants each contributing a tiny effect, plus environmental factors. Polygenic screening is already offered commercially, though reproductive specialists have raised serious concerns about its clinical validity and the ethical implications of selecting embryos for traits shaped by so many interacting factors.24PubMed Central. Polygenic embryo screening: four clinical considerations warrant further attention The predictive power of current polygenic scores is modest for any individual, and applying them to a choice between a handful of embryos adds further statistical limitations.
How Regulation Varies Around the World
There is no global consensus on what embryo screening should be used for. A review covering 19 countries found a spectrum of policy approaches ranging from restrictive to permissive, with wide variation in whether PGT is publicly funded, which conditions qualify, and who decides.25PubMed Central. Regulating Preimplantation Genetic Testing across the World: A Comparison of International Policy and Ethical Perspectives In Europe alone, there is substantial variation: some countries ban PGT entirely for certain uses, others permit it only for conditions deemed serious, and the definition of “serious” itself differs from one regulatory body to another.26Reproductive BioMedicine Society Online. Comparative preimplantation genetic diagnosis policy in Europe and the USA and its implications for reproductive tourism
The UK, for instance, maintains a pre-approved list of genetic conditions for which PGT-M may be offered but also allows case-by-case applications for unlisted conditions. Japan relies on a definition statement of disease severity. Western Australia uses a list of considered factors reviewed on a case-by-case basis.27PubMed Central. Evaluating standards for ‘serious’ disease for preimplantation genetic testing: a multi-case study on regulatory frameworks in Japan, the UK, and Western Australia The United States, by contrast, has virtually no federal regulation of PGT. Clinics can offer screening for nearly any condition or trait without government approval, which is one reason commercial polygenic screening has gained a foothold there faster than in more regulated markets.
This patchwork of rules has given rise to reproductive tourism, where patients travel to countries with more permissive regulations to access testing unavailable at home. Sex selection, for example, is banned in many countries but readily available in the United States. The lack of harmonized international standards means that access to embryo screening depends heavily on where you live and what you can afford, not just on what the science can do.