After a woman has her eggs retrieved, there are typically two options when it comes to the timing of a transfer. The first is a “fresh” transfer, whereby the best embryo(s) are transferred into the uterus only three to five days after the retrieval. Any remaining embryos are frozen for a future transfer, if needed.
Alternatively, all the embryos could be frozen immediately, including the best embryos, to be thawed for a transfer at a later date.
In many cases it will be clear before the cycle even starts that some women will need to do a frozen transfer (for example women who have PCOS or those who want to do PGS testing). But in other cases that will only be revealed right before the retrieval (like when a women has high progesterone at the time of retrieval).
Below we summarizes the trade-offs for both approaches.
As you may recall from other lessons and chapters, nearly all IVF cycles involve stimulating the ovaries to retrieve a high number of eggs. A result of creating so many eggs is that estrogen and progesterone levels can rise 10 times higher than their normal peak levels. The uterus takes note of such a rise and hurries to prepare itself to receive an embryo. Therein lies a problem. That’s because the uterus is only “receptive” to an embryo implanting for a short time, and when it rushes to prepare itself in reaction to the hormonal surge, this “window of receptivity” can be moved earlier.
As a result, in a fresh cycle, by the time an embryo is mature and ready for transfer, the uterus is less likely to be “receptive” and the odds the embryo implants are lower. Conversely, in a frozen cycle, the doctors wait for a woman’s hormonal balance to restore and can transfer the embryo on a later date, at a moment when her uterus is most receptive.