The implantation of a fertilized egg, already called an embryo, is the process that occurs when the embryo attaches to the endometrium (the inner layer of the uterus). In an IVF cycle, embryo implantation occurs after transfer and marks the beginning of intrauterine embryonic development and the first symptoms of pregnancy.
The different sections of this article have been assembled into the following table of contents.
Embryo implantation is the moment when the fertilized egg is detached from its sheath (zona pellucida), adhered to the endometrium and anchored to it to begin its intrauterine development.
The embryo that performs the implantation is in blastocyst stage, which is reached between 5 and 6 days after fertilization. At this stage of embryonic development, the blastocyst has approximately 200-400 cells, differentiated into two different cell types:
In addition to these two distinct cell types, a central cavity is formed known as a blastocele. In this image we can see the structure of the blastocyst, i.e. the embryo prepared for uterine implantation.
For implantation to take place and pregnancy to be achieved, there are two key factors:
These aspects allow for adequate synchrony and interaction between uterus and embryo. Therefore, if good quality embryos are transferred to the woman with a suitable endometrial preparation, the likelihood of achieving pregnancy increases.
The period of implantation of the embryos in the female uterus consists of several phases: detachment of the zona pellucida, pre-contact and apposition, adhesion and invasion. In the following image we can see the stages of implementation, although we explain them in detail below.
The first step for the embryo to implant is to leave its shell: the zona pellucida. It's what's known as hatching. It consists of the breakage of the zona pellucida and the exit of the embryo, both from the ICM and from the trofoectoderm.
Approximately between day 5 and 6 of embryonic development, the fertilized egg is positioned in the endometrial tissue and remains immobile in the acquired position. It only directs the embryonic pole (where the ICM is) towards the epithelium of the endometrium.
In this phase, the so-called pinopods, projections of the endometrial cells that help the blastocyst at the junction with the endometrial epithelium, are fundamental.
This is the moment when the trofoectoderm cells strongly bind to endometrial cells through adhesion molecules such as integrins, L-selectins, proteoglycans, fibronectins, etc.
This usually occurs from day 8-9 of embryonic development.
Little by little, the cells of the trofoectoderm proliferate towards the endometrium and thus manage to displace and replace the endometrial cells. This eventually leads to complete invasion of the endometrial stroma by the trophoblast, which becomes totally embedded in the endometrium.
Implantation lasts approximately 4-5 days, from the time the embryo leaves the zona pellucida (hatching) until the trofoectoderm completely invades the endometrium to initiate the formation of the placenta and allow embryonic development to continue.
Generally, implantation begins when the embryo is 6 days old, that is, about 6 days after fertilization. For the blastocyst-stage embryo to adhere to the endometrium, it is essential that there is adequate communication between the two, which is only possible during the so-called implantation windowwhich spans more or less from day 20 of the female cycle through 24-25.
The main qualities of the receptive endometrium are:
The passage from the nonreceptive to the receptive uterus occurs only under the hormonal influence. For this reason, it is essential that the woman to whom the embryo or embryos are to be transferred receives estrogen and progesterone supplements to allow the endometrium to move from its nonreceptive state to its deciduous or receptive state.
In cases of surrogacy it is the surrogate pregnant woman who will receive hormonal treatment that will allow her endometrium to become receptive and, thus, the embryos transferred can implant.
The implantation of the fertilized egg does not always give rise to specific symptoms by which we can confirm that the embryos have implanted in the uterus.
However, there are women who experience certain symptoms or signs on the days of nesting that may make us suspect that implantation has occurred. Some of the most common are:
These symptoms are a consequence of the hormonal change characteristic of pregnancy, especially strong in the first few weeks after implantation. Therefore, we can say that, with the exception of implantation bleeding and cramps, they are not symptoms of nesting per se, but of early embryonic development (post-implantation).
In the following article you can read more information about this process: Symptoms after embryo implantation.
Embryo implantation in the surrogate mother is exactly the same procedure as in normal IVF cycles. The embryo does not understand if it is in the uterus of the future mother or in the uterus of the surrogate mother. All it needs is for the uterus in which it has been transferred to be receptive, that is, to have the optimum qualities so that it can be implanted in it and continue its development.
Medication administered during fertilization treatment in vitro (IVF) may increase the intensity of symptoms after implantation. Also, the psychological factor plays an important role in this sense, as emotional involvement in IVF treatments can aggravate the symptoms or their sensation.
When we talk about embryo transfer and embryo transfer of frozen embryos we don't refer to the transfer of frozen embryos but to the process of embryo transfer after being frozen and thawn. So, once in the uterus, implantation happens exactly the same way, regardless of whether the embryos have been created in the same cycle or if they come from previous cycles and, therefore, have bean frozen for a certain period of time.
Of course. This is what would result in multiple pregnancy. For example, the implantation of two embryos results in a twin pregnancy, the implantation of three in the gestation of triplets, etc.
In cases of IVF, be it own IVF or IVF in surrogacy, the embryo transfer with the highest amount of embryos facilitates multiple pregnancy.
We have commented that it is necessary for the woman to whom the embryos are transferred to receive a hormonal treatment of endometrial preparation to favour the implantation of the embryos. Do you want to know what this treatment consists of? In this link you’ll get more information: Endometrium preparation
In addition to the proper condition of the endometrium, we have also mentioned that it is essential that the transferred blastocyst be of quality. If you want to know more about embryo quality, I recommend you consult this link: How is the quality of embryos measured?
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