The pregnancy rate using fresh eggs and vitrified eggs from ovodonation are very similar.
The truth is that once the eggs are combined with the sperm (microinjected), the evolution will be the same, whether the eggs are fresh or vitrified:
- Fertilisation 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. Both with fresh and vitrified eggs, unless the semen is severely altered, a high percentage of eggs will be successfully fertilised (70-80%).
- Arrival at blastocyst
- 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 eggs as with 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 also show no difference.
- Clinical gestation rate and live birth
- similar results will also be seen in the most important outcome, the percentage of transfers that result in a baby. This means that the chance of miscarriage is not increased.
However, the only difference might be in the number of initial eggs. Why? Well, because with vitrified eggs an additional procedure is necessary, which is the devitrification (or thawing) of the eggs. This means that we may lose some eggs that do not survive devitrification. By starting from a lower number of eggs, the number of embryos, and therefore the cumulative gestation rate, could also decrease.
According to the 2017 report of the Spanish Fertility Society, in the total of national centres, an oocyte survival rate of 87.5% was obtained from donor oocytes. In the leading laboratories in these procedures, the oocyte survival rate can be close to 100% (around 97%).
Therefore, it is essential that the laboratory has very high survival rates when working with vitrified oocytes. A few years ago, when the technique was not yet perfected, many oocytes were lost with thawing or devitrification. Today, however, most laboratories have very high oocyte survival rates.
Thus, we could conclude that what is important with vitrified eggs is the percentage of survival to devitrification. Once this step has been passed, the chances of pregnancy will be the same as with fresh eggs.
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