The most frustrating problem in IVF today is that of failed implantation. While we have become very good at creating top-quality embryos in the lab and transferring them into the uterus, we still cannot ensure they will implant. This is because implantation is a biological process over which we have not control. Because we can't track what happens in the uterine cavity after we transfer the embryos, we really don't have a very good answer for why embryos don't implant for a particular patient, which means we are groping in the dark.
One of the options which can help to improve the probability of success is called embryo- pooling. This means we generate many top quality blastocysts over 2-4 fresh cycles; freeze them; and then, transfer them one by one, over a period of time , either in a natural cycle , or one in which the endometrium is prepared using estrogen.
Now this might seem complicated and expensive, but it helps to increase pregnancy rates. The reason for this is that the implantation rate of a top-quality embryo in a fresh cycle is less than it would be in a natural cycle . This is because when we super-ovulate the patient with hormonal injections in order to grow many eggs for IVF, our focus is on producing lots of good-quality eggs. This means that the supraphysiological high levels of hormones which the endometrium is exposed to may cause its receptivity to become suboptimal, so that even though it looks perfect on a vaginal ultrasound scan, the endometrial receptivity may not be optimal.
One way of bypassing this problem is not to transfer in the fresh cycle, but to freeze all the embryos. The beauty is that when we use vitrification in a good clinic, the survival rate is practically 100%, so we don't lose any embryos ! We can then transfer these blastocysts , one at a time , in a frozen thaw cycle , when the focus is purely on preparing the endometrium for optimal receptivity . This allows the patient to get the best of both worlds, so that the pregnancy rate becomes higher.
This means women can do multiple fresh cycles; freeze the embryos; and then come back later on for transferring these embryos , one at a time. This is specially useful for older women , because time is at a premium for them. It makes sense for them to generate lots of embryos and freeze all of them, so they can transfer them sequentially.