What is blastocyst culture?
Blastocyst means the stage of the human embryo that is reached about five to six days after fertilization. The life span of a human embryo in vitro starts from day 1 at one cell, day two at two to four cells, day three at eight cells, day 4 at 16 cells and compacted and day five at blastocyst stage (at this stage the embryo’s appearance changes into a fluid-filled cavity with an inner cell mass that will grow into a human and more than 100 cells around the cavity that will grow into the placenta). The time-lapse video shows the development in vitro of a human embryo from day one to day 5–6.
The difference between blastocyst culture and normal culture
In the past, the embryo stage that is common for transfer is the four-to-eight-cell stage (2–3 days). By culturing the embryos to the blastocyst stage, the duration of culture will be extended to at least five days. By doing this, the information of the embryos’ development can be observed for longer.
For more information of the in vitro development of human preimplantation embryos, we have found that not all fertilized eggs can be developed in the same way until they reach the blastocyst stage. Some of them will arrest development at only two to eight cells and die later. Only some of them can grow beyond the eight cells and develop into blastocysts. There are a few factors that impact the ability to grow to blastocysts. The first factor is the good quality control of process of the in vitro fertilization and embryo culture system. In a situation in which we have controlled a good environment and process, the only factor that still exists is the normality of the eggs and sperm, which is the main cause of arrested development. Hence, blastocyst culture is a method of natural selection whereby only embryos with good potential for implantation are chosen and abnormal ones are not selected. However, blastocysts still have the chance of chromosome abnormality that cannot be identified from know from its appearance.
The advantage of blastocyst culture compared with cleavage stage (four-to-eight-cell stage)
Obviously, the chances of implantation of one blastocyst is higher than one eight-cell-stage per embryo transfer. If we need to increase the chance of cleavage stage embryo transfer, we need to increase the number for transfer, consequently increasing the chance of a higher order multiple pregnancy. As blastocyst culture, can help to identify which embryos have the best chance for implantation, the number of embryos that we replace into the uterus can be reduced to one or two without compromising the chance of pregnancy. By doing blastocyst culture and transfer, the chance of a higher order multiple pregnancy will be low and the complication of pregnancy will also be minimized.
The disadvantage of blastocyst culture
Some couple might have the risk of having no embryos for transfer because all embryos are arrested and do not develop into blastocysts. Anyway, if we transfer the embryo before time (on day 2–3), there would also be no pregnancy because when we transfer cleavage stage embryos into the uterine cavity, they will grow into blastocysts in the uterine cavity before implantation. If they cannot grow in vitro, the chances of them developing in vivo would be very low. For the good sight of this, if there is no blastocyst available for transfer, the couple will avoid the unnecessary stress of a two-week wait for results after transfer.
Blastocyst culture for all
We have more than ten years’ great experience in blastocyst culture. The chance of blastocyst development depends on the age of the woman, and egg and sperm quality. For our own data, the blastocyst formation rate is about 50–60% in women younger than 35 years old. The rate will be lower for women older than 35 and lowest in those older than 40 years old. We have set the minimum number of fertilized eggs that we still have blastocyst for transfer at five. If we have less than five fertilized eggs we will freeze them at the one-cell stage (two pronuclear stage) and transfer at eight-cell-stage embryos later. The reason to freeze at one-cell stage is the survival rate is as excellent as blastocyst stage. By doing this, the chance of having no embryos for transfer is less than 5%.