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Embryo implantation and pregnancy

implantación embrionaria

The relationship between the embryo and the endometrium is one of the most beautiful processes that occur inside the human body.

The endometrium is the tissue that lines the uterus, where the embryo attaches itself until it penetrates into the lining of the uterus to continue its development. This process occurs after fertilization of the egg in the laboratory (ICSI) and subsequent culture. Once the embryo reaches the blastocyst stage, it is transferred to the uterus. In assisted reproduction, embryos are classified taking into account their quality and other parameters such as symmetry, size, or cellular fragmentation. Embryos can be A, B, C, and D, with A/B qualities being the most optimal for embryo implantation.


The following are the “steps” taken by the embryo as it penetrates into the maternal uterus:


About 5 or 6 days after the embryo transfer, the embryo hatches, shedding its protective outer layer, and implants in the endometrium. At this moment the journey begins: pregnancy.


About seven days later, it loses the zona pellucida and the trophoblast located at the embryonic pole makes contact with the compact layer of the endometrium. It is here that its relationship with the uterus begins.


When the embryo comes into contact with the uterus and other cells that invade the endometrium, it starts to come into contact with the maternal blood. It is during this last phase when a “rupture” of the endometrium occurs and new connections are formed for proper adhesion to the uterine wall, allowing it to continue its development as a fetus.

During embryo implantation, the woman can experience some slight changes in her body. One of the most frequent is the so-called implantation bleeding, where the endometrial blood vessels rupture. It is common bleeding, from the rupture of the endometrial blood vessels. This bleeding or spotting is common and does not imply any risk for the correct evolution of the pregnancy.

However, if the embryo does not nest correctly in the uterus, an embryo implantation failure will occur and the pregnancy will not proceed.


In order for the above-mentioned stages of embryo implantation to occur correctly, the mother’s immune system can react in one way or another. Taking into account that half of the genes are paternal and the other half maternal (or in the case of ovodonation or double donation, donor genes), the maternal body must generate an immuno-tolerance in order to accept the embryo, which is an organism genetically different from the maternal cells. For this, immunosuppression occurs, which will be maintained throughout the pregnancy. Sometimes this does not happen, and an immunological intolerance is generated. If this happens, the woman is likely to suffer implantation failure or, later, miscarriage. On the other hand, if the maternal and paternal genes are compatible with each other and the maternal organism has adequate immunosuppression, pregnancy and fetal growth will begin without any problem.

When the immune system does not achieve the immunotolerance mentioned above, it recognizes the embryo as foreign, and antibodies are formed which prevent the embryo from implanting. In these patients, there is a high number of Natural Killer (NK) cells, a type of lymphocyte with the capacity to destroy organisms not recognized as part of the woman’s body, in this case, the embryo.

Another immunological response that can occur during this phase of implantation can be antiphospholipid syndrome. There are some women with the presence of certain elevated antibodies that cause hypercoagulability, which leads to the formation of thrombi in the placenta and to the loss of the pregnancy, and these patients have a higher risk of thrombosis or repeated miscarriages (which defines this syndrome).

Thrombophilias are also one of the most common causes of embryo implantation failure or recurrent miscarriages. If the patient suffers from any type of thrombophilia (predisposition to form blood clots), this can occur in the placenta and block the development of the fetus.

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