All infertile patients want to maximise their chances of success in their IVF cycle. If one embryo has a 20% chance of becoming a baby, and if transferring 2 embryos will double that chance, then why not transfer 2 routinely ? And using the same logic, why not transfer 4, to increase the odds of success ?
This seems very logical, but more is not always better ! If an embryo is going to become a baby, then even one is enough ! And if it's not, then even six are not going to work. Most doctors believe that beyond three, we've reached the point of diminishing returns.
Also, transferring more embryos just ends up increasing the risk of having a multiple pregnancy. While most infertile couples will treat a twin pregnancy as a bonus, anything more can be a major medical problem, because of the risks of prematurity.
However, decision making in real life can be much harder . Patients get emotionally attached to their embryos - after all, they have invested a lot of time, money and energy in creating them; and they think of them as being potential babies. To tell patients patients to " discard " their spare embryos can be emotional torture - and many feel that they are abandoning their babies , which is why they'd rather the embryos were transferred back into their uterus. This can also be a major cause of regret when the embryos do not implant. Many patients then fantasise that maybe if they had transferred the extra embryos, that these might perhaps have "stuck"; and they regret their decision to discard these even more acutely.
Counselling can help patients to understand that the trick is to transfer the right number of embryos - not too few - and not too many !