There is much confusion about the ability of frozen embryos to produce pregnancy. On initial inspection, the chance for pregnancy using frozen embryos appears to be lower than the transfer of fresh embryos. On closer analysis, however, this may not be true.
Typically, when a frozen embryos transfer is performed, the embryos that were frozen were those that were not chosen for the initial fresh embryo transfer. Why not? Often, the embryologist chose the “best” embryos for the initial transfer. The embryos that remained were frozen. Some programs are very strict about the quality of the embryos they will freeze, other programs will freeze any and all remaining embryos. As in the case of fresh embryo transfer, embryo quality has a profound effect on the chance for pregnancy. If poor quality embryos were frozen, the survival rate after thawing as well as the pregnancy rate would be expected to be lower.
A second important factor is the number of embryos that are transferred. Take the example of a couple who produce four good quality embryos. Two are transferred in the fresh in vitro fertilization cycle and two are frozen. A pregnancy results and the couple delivers a baby. Later, the couple decides to attempt pregnancy again using the frozen embryos. Upon thawing, however, only one embryo survives. The frozen transfer is of a single embryo. Pregnancy rates of a single embryo are going to be lower than that seen with a multiple embryo transfer.
Studies have been performed in women who had a very vigorous stimulation of their ovaries and were considered to be at high risk for ovarian hyperstimulation syndrome (OHSS). Their physicians decided to skip the embryo transfer and instead freeze all of the embryos for later use. This significantly decreases the risk of serious or complicated OHSS. In other cases, the physicians decided to go ahead with a fresh embryo transfer anyway, despite the risk of OHSS. In this experiment, the couples in both groups have a large number of embryos and the selection of embryos for transfer was for the best embryos in each case. These studies show no difference in the pregnancy rates between fresh or frozen embryos.
We can conclude that cryopreservation does not by itself decrease the chances for pregnancy, rather it is the number of quality of embryos available that is the determining factor. Read more about frozen embryo transfer.
In 2006, researchers from New Jersey compared the pregnancy rates obtained when embryos were frozen for different lengths of time. The data showed that no difference in the chance for pregnancy was evident even when embryos were frozen for more than ten years.