Here is a guide to know why the embryo is cultured until day five — till it becomes a blastocyst — and then transferred on the same day
By Dr Sulbha Arora
Going for an IVF treatment? You must educate yourself about embryo transfer and it is done on day five, when it becomes a blastocyst.
A blastocyst is a name given to a five-day-old embryo. The day eggs are removed from the woman’s body, it is considered day zero. From here, once the eggs are fertilized with the sperms, the development is monitored in the laboratory, up to the fifth or sixth day, when it reaches the blastocyst stage.
On day zero, all the eggs may not be mature. Some may be immature eggs or M1 eggs. When ICSI (intracytoplasmic sperm injection) is done, each M2 egg is injected with a healthy sperm, and the M1 eggs are left alone. When conventional in-vitro fertilization (IVF) is done, the sperm sample is added to the petri dish containing eggs and the sperms penetrate the eggs naturally.
On day one: A fertility check is done to find out how many of these eggs have successfully fertilized. Some eggs may not have fertilized at all. Whereas, some eggs may show abnormal fertilization. Then, further progress of the fertilized eggs will be observed.
On days two and three: The embryos reach the cleavage stage; that means the single-cell egg now starts dividing. On day two, around two to four cells are expected to be seen in the embryo. By day three, there are six-eight cells in the embryo, and most of the eggs will reach this stage of development. Earlier, embryo transfer was carried at the cleavage stage. But this can be problematic, because once most of the embryos reach here, it is challenging to pick one or two for transfer.
If only one or two embryos are picked in order to reduce the chances of triplets, quadruplets, and higher-order multiple pregnancies, you may pick embryos that look best under the microscope, but they may not have the growth potential any further beyond this stage. Then, you may have low chances of getting pregnant. The other option is to transfer multiple embryos at this stage (maybe three-four). Here, your chances of pregnancy may increase, but simultaneously, your chances of higher-order multiple pregnancies will increase, too. Hence, it is not an ideal option. Wait till day five, the blastocyst stage, to transfer the embryos.
The advantages of blastocyst stage
Not every embryo that has reached the cleavage stage will reach the blastocyst stage. So, a natural selection already takes place at this stage. Some embryos may have stopped growing at any point before reaching the blastocyst stage. If you test these arrested embryos, you will find the majority that got arrested or stopped growing, would have had chromosomal problems or aneuploidies, and nature did not allow them to grow further.
During natural selection, only the strongest and potent embryos will reach the blastocyst stage on day five. By doing the embryo transfer at the blastocyst stage, clear selection can be made about which one or two embryos have the best potential for growth.
* Firstly, in a fresh cycle, when the egg pickup is done on day zero, it is postulated that there are still contractions occurring in the uterus which may last up to two-three days. By the time one’s embryos reach the day five stage, it has already been five days after the egg pick up procedure and the uterine musculature is more relaxed. So, the chances of the embryos getting expelled out of the uterine cavity are lower.
* Secondly, according to the natural physiology in the woman’s body, the egg is released by the ovary and picked up by the fallopian tube. The fertilization happens in the fallopian tube and the embryo grows up to the cleavage stage that is day two or three stage in the fallopian tube. It only starts moving towards the uterine cavity at the blastocyst stage. If you put cleavage-stage embryos in the uterus, you are putting them in the unnatural environment as the ideal environment is in the fallopian tube. The ideal environment for blastocyst is the uterus.
Lastly, the growth of the embryo from the egg stage to the cleavage stage depends more on the egg factor. After the cleavage stage, the embryonic genome switch takes place and the growth of the embryo from cleavage stage to blastocyst stage depends more on the sperm factor.
By observing the growth of the embryo till the blastocyst stage, it is possible to gain more information about its development. In case of no viable embryos or poor quality embryos or a negative pregnancy test, it can help to determine if there is an issue with the egg or the sperm factor, although the exact cause may not always be pin-pointed with certainty.
(The writer is a clinical director, Nova IVF Fertility, Mumbai)
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