Embryo transfer typically occurs between Day 2 and Day 5 after egg retrieval. The timing depends on the embryo development rate and the clinic's protocol.  

 Why Does Timing Matter?  

- Longer culture times (up to Day 5) lead to embryo loss, but this is often due to natural selection—chromosomally abnormal embryos are less likely to reach the blastocyst stage.  

- Some clinics transfer only on Day 5, while others perform transfers on Day 2 or 3.  

 What Determines the Transfer Day?  

- Number of available embryos:  

  - If many embryos are available, clinics may extend culture to Day 5, allowing natural selection to identify the strongest ones.  

  - If few embryos are available, earlier transfer (Day 2 or 3) may be preferred to avoid the risk of no embryos surviving to Day 5.  

- Laboratory strategy:  

  - Extended culture (Day 5 transfer) improves success rates by selecting the best embryos, reducing unnecessary transfers.  

  - However, it carries the emotional risk of losing all embryos before transfer, which can be difficult for some couples.  

Ultimately, the best transfer timing is decided case by case, balancing embryo quality, number, and clinic preferences.