The pregnancy rate using fresh eggs and vitrified eggs from egg donation are very similar.
What is certain is that once the eggs meet the sperm (are microinjected), the evolution will be the same, whether they are fresh or vitrified eggs:
- Fertilization rates
- the day after the microinjection, two pronuclei must be observed in the egg. This means that egg and sperm have mixed their genetic material. With both fresh and vitrified eggs, unless the semen has severe alterations, a high percentage of eggs will present a correct fertilization (70-80%).
- Arrival to blastocyst stage
- not all eggs will develop until day 5 (blastocyst stage). The percentage of eggs that divide and reach this stage of development is the same with fresh and vitrified eggs. This means that the number of embryos obtained will be the same with both types of eggs.
- Implantation rate
- the percentage of embryos that will give a positive pregnancy test after transfer will not show any difference either.
- Clinical gestation rate and live newborns
- Similar results will also be seen in the most important outcome, the percentage of transfers that result in a baby. This means that the possibility of abortion is not increased.
Now, the only difference might be in the initial number of eggs. Why? Because with vitrified eggs an additional procedure is necessary which is the devitrification (or thawing) of the eggs. This implies that we may lose some eggs that do not survive such devitrification. When starting from a lower number of eggs, the number of embryos and therefore the rate of accumulated gestation could also decrease.
Therefore, it is essential if we work with vitrified oocytes that the laboratory has very high survival rates. A few years ago, when the technique was not yet perfected, many oocytes were lost with thawing or devitrification. However, today most laboratories have very high oocyte survival rates.
According to the report of the Spanish Society of Fertility for the year 2017, in the total of national centres a percentage of oocyte survival from donor eggs of 87.5% was obtained. In the most advanced laboratories in these procedures, the oocyte survival rate may be close to 100% (around 97%).
Thus, we could conclude that what is important with vitrified eggs is the percentage of survival to devitrification. Once this step has been overcome, the probabilities of pregnancy will be the same as with fresh eggs.
