These special techniques are used in isolated cases as necessary.

Assisted hatching
This process consists of making a small hole in the zona pellucida (the capsule that protects and surrounds the embryo) so that it can be taken out and implanted in the uterus.

This is usually carried out on older women or after two unsuccessful attempts at pregnancy. The hole is usually made by projecting an acidic fluid directly at the point where we want to make the whole or by laser technology.

Extension of the embryonic culture or sequential culture
The embryo transfer usually takes place on the third day after the extraction of the ova, when the embryos have a middle stage of eight cells.

At times, depending on each case, it may be recommendable to keep the embryos in culture for a few more days (until six days after the extraction) with a view to observing their development and selecting the embryos that are truly viable in order to increase the possibilities of pregnancy. On average, one out of every three embryos continues to develop normally until the sixth day, forming an embryonic structure known as the blastula. This embryo is considered highly evolutionary and usually has an average of between 50 and 100 cells.

Therefore, at this level, the transfer of more than two embryos is never recommended since the multiple pregnancy rate is quite high. On the other hand, the embryos that do not reach this advanced stage (the remaining two thirds) usually stop separating. The pregnancy rate increases slightly in accordance with the transfer of embryos that have been in culture for a lower number of days.

Pre-implantation genetic diagnosis
Nowadays, it is possible to know the chromosomal and genic state of the embryos before they are implanted so that only the 'healthy' embryos with regard to the anomalies studied are placed in the uterus.

This technique is not commonplace but it will probably be routine in the IVF laboratory in the future. In general, the diseases that are usually studied include Down syndrome, Edwards syndrome, sexual chromosome alterations such as Turner syndrome and the more common monogenic diseases such as cystic fibrosis, Duchenne muscular dystrophy, etc.

Through this study, it is also possible to know the gender of the embryos to discard gender-related genetic diseases. Consequently the medical indication for performing a genetic study on embryos is, above all, aimed at discovering traces of any particular genetic disease which could lead to an increase in the risk of the disease appearing in children.

Other reasons for which the preimplantation diagnosis is recommended is after several miscarriages or in the case of older women, since the increase in the risk of chromosomopathies in babies when the mother is over 35 years of age is well known.

The process consists of obtaining embryos through in vitro fertilisation and, once they have reached the 6-8 cell stage, to extract one or two cells for analysis using molecular genetic techniques. Depending on the result of the study, which usually takes 1-2 days, the embryos that are found to be normal for the disease studied are transferred.