How Long Can Embryos Stay in Culture? Understanding Extended Embryo Culture
In the context of in vitro fertilization (IVF), embryo culture refers to the period during which fertilized eggs are nurtured in a laboratory setting before being transferred to the uterus. The duration of this culture plays a pivotal role in the success of IVF treatments. The Guardian features an excellent graphic and explanation illustrating what happens to the development of fertilized eggs after Day 7.
Guardian Graphic
Standard Duration of Embryo Culture
Traditionally, embryos are cultured until the cleavage stage, occurring around day 2 to 4 post-fertilization. At this juncture, embryos typically consist of 4 to 8 cells and are then transferred to the uterus. This approach has been widely adopted due to its simplicity and the shorter time embryos spend outside the maternal environment.
Extended Embryo Culture: Advancing to the Blastocyst Stage
Extended embryo culture involves maintaining embryos in vitro until they reach the blastocyst stage, approximately day 5, 6, or 7 after fertilization. Blastocysts comprise about 100 cells and have a more complex structure, which can enhance the selection process for transfer. This method aims to improve implantation rates by allowing embryologists to identify embryos with higher developmental potential. Check out this systematic review of all studies regarding extended blastocyst culture until Day 7. Even the review notes that most IVF labs do not culture to Day 7, but should they?
Advantages of Extended Culture
Enhanced Selection: Culturing embryos to the blastocyst stage allows for a better assessment of their viability, potentially leading to higher implantation and pregnancy rates.
Synchronization: Transferring blastocysts may better align with the uterine environment's receptivity, potentially improving implantation success.
Considerations and Risks
While extended culture offers benefits, it also presents certain challenges:
Embryo Viability: Not all embryos will develop into blastocysts; some may arrest during extended culture, reducing the number available for transfer.
Developmental Concerns: Some studies suggest that extended culture might be associated with adverse outcomes. For instance, research indicates that children born after frozen embryo transfer (FET) scored lower on certain cognitive assessments, potentially linked to extended embryo culture rather than the freezing process itself.
Emerging Research: Extending Culture Beyond Day 7
Emerging research is exploring the possibility of extending embryo culture beyond Day 7, though current ethical and legal guidelines set the limit at 14 days. This limit, known as the 14-day rule, exists to prevent experimentation on embryos as they begin developing structures critical for implantation and early organ formation. Scientists are particularly interested in whether embryos cultured for longer periods could provide insights into implantation failures, early pregnancy loss, and developmental biology. However, maintaining embryo viability beyond Day 7 remains challenging, as metabolic demands increase, and the culture environment may not fully replicate the conditions of the uterus. As technology advances, this research could reshape our understanding of embryonic development while raising important ethical considerations about the future of assisted reproduction.
Now before you go and question your IVF lab for not culturing past 7 days, the reason they don’t do that is because embryos have different culturing conditions after that 7 day period. A standard IVF lab will not be able to manage a longer culturing period and it will likely lead to the embryos arresting or degenerating. Extended embryo culture is not as easy as it looks and actually requires significant materials and conditions in order to be successful!
Check out this Times article that was published in March 2025 about extended culture on human embryo research! Click me!
Reference - Marta Shahbazi/Zernicka-Goetz Lab
Ethical and Legal Considerations
It's important to note that legal and ethical guidelines regulate the maximum duration for which human embryos can be cultured. In many jurisdictions, including the UK, the limit is set at 14 days. Recently, there have been discussions about extending this limit to 28 days to facilitate research into early developmental stages, which could enhance understanding of miscarriages and congenital conditions.
Many scientists have tried to protest these rules with no success. Do you think the government should extend the time limit or do you think it will lead to unintended consequences?
Conclusion
The duration of embryo culture in IVF is a critical factor influencing treatment outcomes. While extended culture to the blastocyst stage offers advantages in embryo selection and synchronization with the uterine environment, it also requires careful consideration of potential risks and ethical guidelines. Ongoing research continues to refine these practices, aiming to optimize success rates while ensuring the safety and well-being of resulting offspring.
Hey, you never know! We will say that some labs already will refuse to culture embryos to day 7 because they don’t think those embryos will be viable. On the other hand, many clinics do culture to day 7 with success, but equal amounts of failure. There is no easy answer and it will always come down to the clinic and patients!
“Embryos in extended culture are like houseguests—stay just the right amount of time, and everything’s great. Stay too long? Things start getting weird.”
