How Many Embryos To Transfer At Once

The primary goal of an embryo transfer is to achieve a healthy, singleton live birth. The number of embryos transferred directly influences the probability of success, but the relationship is not always straightforward.

Key Factors Influencing Success Rates:

1. Embryo Quality and Ploidy Status (The Most Critical Factor):
   • Euploid (Genetically Normal) Embryos: These embryos have the correct number of chromosomes and are far more likely to implant and develop into a healthy pregnancy. The live birth rate for a single euploid embryo is typically 50-70% per transfer. Transferring two euploid embryos does not double this success rate but drastically increases the chance of a twin pregnancy to over 50%.
   • Untested Embryos: Without preimplantation genetic testing for aneuploidy (PGT-A), embryo selection is based on morphology (grading). While a well-graded “good quality” untested embryo has a good chance, the risk of aneuploidy (a genetic abnormality that causes implantation failure or miscarriage) is significant, especially with increasing maternal age. This is why clinics historically transferred more untested embryos to compensate for potential failures.
2. Patient and Surrogate Age (at time of egg retrieval):
   • The age of the egg provider is the strongest predictor of embryo euploidy. For example, eggs from a 30-year-old have a much higher rate of genetic normalcy than eggs from a 43-year-old. This is a fixed variable in surrogacy but is crucial for setting expectations on embryo quality and the subsequent transfer strategy.
3. Surrogate Uterine Receptivity:
   • A healthy, well-prepared surrogate uterus provides the optimal environment for implantation. The success rates quoted assume a surrogate with no underlying uterine factors that could impede implantation.

Data-Driven Conclusion on Success:
For most scenarios, especially when using PGT-A tested embryos, the data strongly supports Elective Single Embryo Transfer (eSET). The cumulative live birth rate from transferring one high-quality embryo at a time is often equivalent to or better than transferring multiple embryos simultaneously, while avoiding the profound risks of a multiple pregnancy. The strategy shifts from “how many to get one pregnancy” to “how to achieve one healthy baby at a time.”