Fish, vegetables, pizza…almost everything is better when it's fresh. Embryos, however, seem to be an exception to that rule. So much so that most of our IVF cycles these days are “freeze-all”: we skip the fresh transfer, vitrify the viable embryos, then defer the transfer to the next cycle.

Look at our data for the last 12 months (all age classes combined):

Note that 90% of our transfers are frozen, and under 10% are fresh. Why so many frozen transfers?

First of all, safety: the elevation in hormones after ovarian stimulation can be exacerbated if a transferred embryo implants, leading to the potentially dangerous complication called ovarian hyperstimulation syndrome (OHSS). Delaying the transfer by vitrifying the embryos gives the hormones a chance to clear the system, thus avoiding the risk of OHSS.

Secondly, check out the pregnancy rates: frozen rates are consistently higher. So clearly freezing the embryo somehow makes it better, right? Not exactly. Freezing does not improve the embryo, but it does give us an opportunity to create a better environment for that embryo. Unlike the very high estrogen conditions that are unavoidable after ovarian stimulation, a more physiological situation is created during the frozen embryo cycle, allowing better receptivity of the endometrium.

(Thirdly, about ½ of our frozen embryo transfers are for patients who are pursuing preimplantation genetic testing for aneuploidy (PGT-A), but that’s a topic for another day…)

Are there any specific disadvantages of a frozen embryo transfer? 1) There is a risk that the embryo won’t survive vitrification, but it is far less than 1%. 2) The frozen embryo transfer adds about 1 month to the process compared to a fresh transfer.

Are there greater risks to the mothers or babies after frozen embryo transfer? Important question: a recent meta-analysis asked if frozen embryo transfers were better for mothers and babies. The aggregate data from 26 studies that were included demonstrated that pregnancies after frozen transfer had lower risks for preterm delivery, low birth weight, and small size at birth. However, there were higher risks for high blood pressure during pregnancy, high birth weight, and large size at birth. There were no differences in birth defects.

“SO bottom-line it for me, Doc,” you demand. “What’s better? Fresh or frozen?”

The answer is: it depends, but when in doubt, I usually favor freezing. On balance, frozen embryo transfers can increase safety and improve pregnancy rates compared to fresh transfers.

On the other hand, in many cases fresh transfers are perfectly reasonable and successful too. What is right for you really depends on your unique case.