Blastocyst culture is a technique that allows the selection of embryos with proven potential of further development, given that only the best embryos reach the blastocyst stage.
Embryos developing following fertilisation and subsequent cell division reach the blastocyst stage on the 5th – 6th day after fertilisation. The blastocyst consists of 70-100 cells (while 3rd - day embryos consist of 8 cells). It contains 2 cell groups, the embryoblast on the inside and the trophoblast on the outside. The embryoblast is a cell mass which will develop into the embryo, while the trophoblast is the outer layer, which will form the placenta.
Thus, blastocyst culture includes embryo culture until the 5th day following oocyte retrieval. This method is recommended to couples with sufficient embryos (more than 6-8), at least 4 of which have reached the appropriate stage of development and consequently are of the appropriate quality on the 3rd day after oocyte retrieval. In the embryology laboratory of “gennima”, as well as in other reliable embryology laboratories, at least 30-40% of embryos reach the blastocyst stage; e.g. if a couple has 12 oocytes following oocyte retrieval, 8 of which are fertilised, 4-6 good quality embryos are expected on the 3rd day, while 2-4 blastocysts are expected to have developed on the 5th day.
Due to its numerous advantages, this method is recommended to couples with sufficient embryos allowing blastocyst culture.
• It allows the selection of the best embryos
During embryo culture until the 5th day, better quality embryos (based on international criteria of embryologists), i.e. those with the best potential of achieving pregnancy, stand out. Thus, there is a possibility of transferring fewer embryos (usually 2) of the most appropriate ones for achieving pregnancy.
• It reduces the risk of multiple pregnancy
The fact that fewer embryos (up to 2 blastocysts), with great potential of pregnancy achievement, are transferred, increases success rates, while it also reduces multiple pregnancy risk. Thus, chances of giving birth to a healthy baby are increased and the risk of preterm labour and low birth weight is avoided.
Sometimes (although not too often) the phenomenon of blastocyst division is observed; then, identical twins develop from 1 blastocyst. In case of blastocyst transfer, with one of the two blastocysts dividing, a triplet pregnancy might be finally achieved. For that reason, a low triplet pregnancy percentage is observed. It is worth noting that division is possible in 3rd – day embryos, too. However, this phenomenon is slightly more frequent when blastocysts are transferred.
• The endometrium seems to be more receptive on the 5th day following oocyte retrieval. After oocyte retrieval, uterine contractions are observed due to increased hormone levels. In fact, in some patients, these contractions are quite intense and feel like mild menstrual pain. Contractions tend to subside after the 3rd day from oocyte retrieval. Thus, if blastocyst transfer is performed on the 5th day, less contractions and more favorable conditions at embryo transfer are expected. Moreover, on the 5th day, less cervical discharge is observed – which facilitates embryo transfer.
• Embryo culture until the 5th day provides us with more information on embryo quality.
With blastocyst culture, the doctor has a more accurate outlook on the couple’s infertility. Sometimes, 3rd – day embryos that seem to be of top quality, develop into mediocre quality embryos. Thus, embryos aren’t of really good quality, which explains the couple’s infertility, but also hinders pregnancy achievement. Of course, this doesn’t mean that pregnancy will not be achieved, but the couple may finally face more difficulties than initially anticipated.
• Allows preimplantation genetic diagnosis.
Embryo biopsy to rule out genetic diseases (e.g. b-thalassemia, cystic fibrosis) or chromosomal abnormalities (e.g. Down syndrome) is performed in 1 of the 3rd- day embryo’s 8 cells. Embryo transfer is performed on the 5th day; by then, embryos have been screened and selected based on test results.
We have elaborated on the advantages of blastocyst culture; does this method have any disadvantages though? Practically not, except the fact that usually fewer blastocysts (compared to the number of 3rd-day embryos) develop and possibly there will be no redundant blastocysts to cryopreserve for future cycles.
However, blastocyst culture isn’t recommended to all couples. If 3rd-day embryos are few, it is deemed preferable to transfer them in the uterus, i.e. their natural environment, where chances of achieving pregnancy are more. Besides, most pregnancies with the aid of IVF have been achieved with 2nd or 3rd-day embryos, as the technique of blastocyst culture is more recent.
At this point, we must stress that not all embryology laboratories can support a successful blastocyst programme. An average laboratory can support embryo culture up to the 2nd or 3rd day. Blastocyst culture requires a laboratory of exceptional quality, ensuring ideal conditions. At “gennima”, we are proud of the high rates of blastocyst culture that our embryology laboratory has, which gives us the possibility to offer the couples who trust us the highest possible success rate (pregnancy achievement rate). Blastocyst transfer increases the possibility of achieving pregnancy, because on the one hand, only top embryos reach the blastocyst stage and on the other hand, these couples had good quality embryos to begin with, which classifies them among candidates with increased possibilities of pregnancy achievement.
Finally, it is worth mentioning that children born by means of blastocyst culture don’t seem to have any difference from children born from embryos transferred on the 3rd day. The following study from Austria (2004) is indicative:
‘Pregnancy outcome after blastocyst transfer as compared to early cleavage stage embryo transfer’
Human Reproduction Vol.19, No.9 pp. 2097–2102, 2004
This study shows no statistically significant differences in gender, Apgar score (scoring system of the newborn’s health right after birth), average birth weight and number of congenital abnormalities.
Other scientific articles concerning blastocyst culture:
‘Live birth rates after transfer of equal number of blastocysts or cleavage-stage embryos in IVF. A systematic review and meta-analysis’
Human Reproduction Vol.23, No.1 pp. 91–99, 2008
‘A prospective randomized study: day 2 versus day 5 embryo transfer.’
Human Reproduction Vol.17, No.6 pp.1507-1512, 2002