Implantation (Facts Everyone Should Know )

Embryo implantation is the process by which the embryo is anchored in the endometrium and is essential for pregnancy to occur. From that moment the placenta will begin to form to allow the nutrition, sustenance and protection of the future baby during pregnancy.

The following video discusses this process, when it occurs and the differences between embryo implantation in cases of natural fertilization

Knowing why some embryos implant and others don’t is one of the challenges of reproductive medicine. The intimate knowledge of the physiological aspects of this process is important to design strategies that increase the chances of pregnancy.

Only one third of cycles considered fertile end in pregnancy. The cause of this low success rate is due to several factors such as an inappropriate uterine environment (since there must be a hormonal environment conducive to estradiol and progesterone), a poor embryo quality , a low receptivity of the maternal endometrium and a defective dialogue between both (embryo-endometrium).

The rate of clinical pregnancy per menstrual cycle in couples considered fertile who maintain regular relationships without contraceptives is 25-30%.

This rate is relatively low compared to other animal species, although it is cumulative, that is, it increases month by month, reaching 80% after 10 months. However, it decreases with the increase in the age of the woman.

Years ago, the treatment of implantation was limited to the use of luteal phase support protocols by the administration of progesterone and hCG ( human chorionic gonadotropin ).

Currently, several basic and clinical studies on this subject have allowed to know the role played in the implantation of different cell populations of the uterus and the role of some molecules such as cytokines or integrins, adhesion molecules and growth factors.

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What is needed for there to be an implantation?

There is evidence that cytokines and growth factors play an important role as local mediators of the actions of steroid hormones on the endometrium, in order to prepare it for implantation.

It is believed that signals from the preimplantation embryo could induce the production of cytokines by the endometrium. These, in turn and by binding to specific membrane receptors, would activate the expression of adhesion molecules such as integrins, responsible for mediating the adhesion of the blastocyst to the endometrium.

Biologically, it is a complex process where a blastocyst embryo is fixed to the receptive endometrium of the mother. There must be a total synchronicity between embryo and endometrium. Therefore, for implantation to occur the following would be necessary:

The fertilization of the ovum by the spermatozoon
The proper development of the embryo up to blastocyst
The journey from the embryo to the uterus
The receptivity of the endometrium
The interaction of the molecules produced by the embryo and the endometrium

When does the implantation occur?

The implantation takes place in the middle third of the posterior face of the uterus and happens during a specific period of time, in which the dialogue between the embryo and the endometrium of the mother is possible.

This moment is called the implantation window and comprises from the 20th day of the cycle until the 24th (around day 6-7 after fertilization) in a standard 28-day menstrual cycle.

The implantation window is the period in which the endometrium is receptive to the embryo and therefore allows adhesion of the blastocyst.

The change from non-receptive to receptive endometrium only occurs if the appropriate hormonal environment exists and if the blastocyst expresses the appropriate molecules to induce it (selectins, cytokines, etc.).

How is the implantation of the embryo?

The implantation or nesting period consists of four phases:

Precontact phase : on days 5-8 of embryonic development the blastocyst takes position on the endometrial tissue. It remains immobile and is oriented with the embryonic pole towards the endometrium to allow later the adequate formation of the placenta.

Apposition phase : the blastocyst seeks the place of implantation, a place to adhere orienting its internal cell mass in the pole in which the trophoectoderm is going to adhere to the endometrial epithelium.

Here the so-called pinópodos (cytoplasmic projections of the endometrial epithelial cells) play an important role, since they help the blastocyst to come into contact with the endometrial epithelium.

It is verified that these pinópodos are clear morphological markers of the endometrial receptivity and only appear during the implantation window, disappearing around the 24th day of the cycle.

Adhesion phase : the blastocyst adheres to the endometrial epithelium, is attached to it by adhesion molecules. This happens 6-7 days after fertilization, the blastocyst already having a diameter of 300-400 μm.

Invasion phase : the blastocyst, more specifically the embryonic trophoblast, invades the endometrial stroma, breaks the basement membrane and penetrates the maternal blood vessels. The trophoblast cells displace, dissociate and replace the epithelial cells, continuing to invade the basement membrane and the underlying stroma.

Throughout this process, cytokines play a very important role, more specifically chemokines or chemokines. Also involved are growth factors and adhesion molecules such as integrins to allow embryo-endometrium contact to be established and finally invasion of the endometrial tissue.

Although at first sight it may seem simple that a blastocyst is implanted in the maternal endometrium, this process is of great complexity and is still not completely known.

Within in vitro fertilization , implantation is one of the most delicate and decisive moments to achieve pregnancy and, since it can not be controlled externally, depends on the circumstances surrounding the patient such as embryo quality, endometrial receptivity, moment of the transfer and total synchrony between all these factors.

Questions from users

What is the difference between fertilization and implantation?
Fertilization is the union between the ovule and the sperm to create a new being: the embryo, which will develop throughout pregnancy. In contrast, implantation is the process by which the embryo enters the endometrium (layer that covers the uterus internally) to be able to form the placenta and thus be able to nourish and grow during pregnancy.

Is the implantation of the egg, the zygote, the embryo or the fetus?
At implantation, the embryo penetrates the endometrium. Once fertilization occurs, we no longer speak of an ovule, but of a zygote. Once the zygote is formed, it divides to give rise to the embryo. As of week 8 of pregnancy, the embryo is called a fetus.

Access this article for more information: Differences between zygote, embryo and fetus .

Do you feel pain in the implantation?
You may notice slight discomfort as in menstruation , but there is no significant pain. Normally the woman does not notice anything and does not realize that the implantation has taken place until the pregnancy test is done and she tests positive.

Can the embryo implant abnormally?
Yes, you can implant in the wrong place, outside the uterus. This is what is called ectopic pregnancy and can occur, for example, in the fallopian tubes.

It is also possible that the embryo implanted correctly in the endometrium but that it can not continue its development, that is, that it leads to a non-evolutionary gestation. This can occur because there is some alteration in the embryo, either in its development or in its genes.

An example of non-evolutionary pregnancy is the biochemical pregnancy , in which the gestational sac is formed but there is no fetus inside. A miscarriage occurs in this type of pregnancy , usually very early.

How can I know if there is an implantation? Are there any symptoms?
To know if there has been implantation it is necessary to make a pregnancy test. It should wait about 15 days from ovulation or have a delay of the rule so that the result of this test is reliable.

The woman can also present pregnancy symptoms such as implantation bleeding.

You can consult this article to know more about the symptoms that can appear after the embryonic implantation: Symptoms of implantation .

Is there any way to favor implantation?
There are a number of tricks that can help improve the chances of implementation. Having a positive mood, reducing stress, maintaining a balanced diet, exercising moderately and avoiding harmful substances such as alcohol, tobacco and caffeine may favor implantation to a certain extent.


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