Prolonged culture is an advanced assisted reproduction technique that allows embryos to develop in the laboratory for a period of five to six days after fertilization, until they reach the blastocyst stage. This procedure gives specialists the opportunity to observe embryo development beyond the third day, improving the selection of embryos to be transferred to the uterus. By opting for blastocyst transfer, the chances of successful treatment are significantly increased.
Prolonged culture to blastocysts
What is a blastocyst?
A blastocyst is the stage of embryonic development reached around day 5 or 6 after fertilization. At this point, the embryo has undergone several stages of cell division and has formed a differentiated structure. The blastocyst consists of two main cell types:
– Inner cell mass (ICM): these cells will give rise to the future fetus.
– Trophoblast: This group of cells will form the placenta and other structures necessary for fetal development.
This stage is crucial because the embryos have passed the early stages of development and have differentiated into their main components. Blastocysts are more competent to implant in the uterus, which increases the success rate of the treatment.
Preembryo
After 16-20 hours after fertilization (Day 1)
Embryo
From 4 cells 48 hours after fertilization (Day 2)
Embryo
From 8 cells 72 hours after fertilization (Day 3)
Embryo
Compacting 80 hours after fertilization (Day 3)
Embryo
Blastocyst stage 120 hours after fertilization (Day 5)
How is extended cultivation performed?
The extended culture process up to the blastocyst stage is carried out under highly controlled conditions in the laboratory. After fertilization (by in vitro fertilization or ICSI), the embryos are placed in specific culture media that mimic the natural conditions of the human body. These culture media are designed to provide the nutrients necessary for the embryos to develop properly.
During the first three days, embryos divide and go through several stages of development (from 2 cells to 8 cells). However, some embryos may become blocked in their division around the third day, which prevents their further development. With prolonged culture, natural selection is achieved, as only embryos that reach the blastocyst stage continue their development and are considered more viable for transfer to the uterus.
Advantages of extended culture to blastocysts
1. Improves implantation and pregnancy rates:
By allowing embryos to develop to the blastocyst stage, specialists can select those that have demonstrated a greater capacity for survival and development. These embryos have a greater chance of successfully implanting in the uterus, which increases implantation rates and, consequently, pregnancy rates.
2. More accurate selection of better quality embryos:
Observation of embryonic development up to day 5 or 6 allows a more natural selection. Those embryos that have reached the blastocyst stage are more likely to be genetically normal and viable. This translates into greater transfer efficiency, reducing the need to transfer multiple embryos and decreasing the risk of multiple pregnancies.
3. Reduced risk of multiple pregnancies:
By transferring good quality blastocysts, the success rates per embryo transferred are higher. This allows doctors to transfer a smaller number of embryos (even just one) with a higher probability of success, thus decreasing the risk of multiple pregnancies, which carry more risks for both the mother and the fetuses.
4. Better synchronization with the endometrium:
Embryo development in culture to the blastocyst stage is more synchronized with the receptivity of the endometrium on day 5 or 6 of the cycle. This improves the implantation rate, as the blastocysts are ready to implant in the uterus when the endometrium is most receptive.
5. Opportunity for preimplantation genetic diagnosis (PGD):
Blastocysts are ideal for preimplantation genetic diagnosis (PGD) techniques in cases where this is necessary, as they allow more cells to be obtained for analysis without compromising the viability of the embryo. This is especially useful in cases where there is a risk of inherited genetic diseases.
Requirements for extended culture to blastocysts
Prolonged culture to blastocysts is a technique that is not always applicable to all cases. For it to be feasible, it is important that an adequate number of good quality embryos are obtained after fertilization. This is because not all embryos achieved on day 3 will survive to the blastocyst stage, as some will not overcome the cell division blocks.
In cases where a small number of embryos are obtained (e.g. 1 or 2), it may be more advisable to perform the embryo transfer on day 3, as the risk of losing the few embryos available is not justified. However, when the number of embryos is sufficient, prolonged culture is an excellent option to maximize the chances of successful treatment.
Additional considerations
1. Success rates:
Although not all embryos develop into blastocysts, those that do have a higher probability of successful implantation. Blastocyst transfer success rates are generally higher than for day 3 embryos.
2. Cryopreservation of blastocysts:
If more good quality blastocysts are obtained than are needed for the initial transfer, they can be cryopreserved for future use. Blastocyst cryopreservation offers high success rates in subsequent cycles, allowing future transfers to be performed without the need for repeat ovarian stimulation.
3. Additional cost:
Prolonged culture may involve additional cost compared to earlier day embryo transfer, as it requires more laboratory time and follow-up. However, this cost is offset by higher success rates and reduced risk of multiple pregnancies.
