When performing an IVF cycle, our goal is to transfer one or two high-quality embryos to the mother's uterus. To do this we need to obtain an adequate number of eggs from about 6 to 15 depending on the patient characteristics and her treatment response. We need a relatively high number due to the fact that not all eggs will fertilise and those which do fertilise, not all will develop appropriately in the laboratory during the early stages prior to the transfer. In general the average number of embryos suitable to transfer is close to one third of the eggs originally obtained.
However, the percentage of eggs that fertilise and develop properly is extremely variable and sometimes fertilisation rates are high and the development of embryos in the laboratory is optimal. In these cases several embryos are suitable for transfer on the day of transfer. Nevertheless we can only use one or two embryos for the transfer. The rest of the high-quality embryos are "cryopreserved", i.e. they are frozen in order to be used in future transfers while retaining much of their potential to achieve pregnancy.
Embryo freezing has been part of the IVF process since the first treatments began but unfortunately the results were very poor on embryo survival and thus pregnancy rates were very low. This was mainly due to the formation of crystals in the interior of the embryo often leading to its structure being damaged and resulting in a very poor prognosis. In recent years cryobiology has made substantial progress as well as the new techniques used to preserve embryos. These techniques are known as "vitrification" and prepare the embryos with high concentrations of protective substances that make the liquid medium prevent the formation of intracellular ice.
The survival rates obtained by vitrification after thawing is over 90%. The embryo keeps intact almost all their chances for correct implantation.
Vitrification is an ultra-rapid freezing technique based on the use of very high concentrations of cryoprotector and extremely high rates of cooling, which prevent ice crystals from forming. Its implementation as a routine technique in laboratories has contributed to a spectacular improvement in results, compared to other techniques traditionally used, as well as an increase in survival rates to 90%.
Although the results for frozen embryo transfers are a bit lower than those for fresh embryo transfers, advances in freezing techniques are making the difference less marked. Furthermore, embryo freezing does not correlate with a higher risk of suffering anomalies or complications during pregnancy when compared to the general population.
Treatment with frozen embryos is simple, convenient and economical. It does not require multiple daily injections or ultrasound scans. There is no need for sedation and there are no side-effects in the vast majority of cases. This is due to the fact that hormone levels during preparation will be very similar to those of a normal cycle.
Treatment is based on the preparation of the uterus of the mother at the time of thawing and transfer of the cryopreserved embryos. For this, the treatment involves the administration of pills or patches for a period of about two weeks prior to the embryo transfer.
Preparation begins at the start of the menstrual cycle and there is an ultrasound scan about ten days later to confirm that the uterus meets the right conditions before programming the thawing of the embryos and then their subsequent transfer.
Once the transfer has been programmed, the patient begins progesterone vaginally 3 to 5 days before the thawing date.
The embryo transfer technique is identical to that used for fresh embryos. The previous recommendations are also identical and must be continued (either in patch form or vaginal tablets) as least until the pregnancy test.
The preparation is therefore very simple: patches applied to the skin for a few days, ultrasound scans and then the administration of progesterone and embryo transfer.
In recent years with the introduction of vitrification, embryo survival rates and pregnancy rates have improved considerably and have even exceeded the most optimistic viewpoints.
Keep in mind that the chances of achieving pregnancy in IVF vary greatly depending on the type of treatment and the partners' characteristics (age, cause of infertility, etc.). The success rates are individual to each case. The success rate is slightly lower than that obtained with fresh embryos.
The data on pregnancies and children born after frozen embryo treatments shows no difference to those using embryos transferred without prior cryopreservation. Therefore there is no evidence against the performing of this type of treatment.
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