An essential stage in the beginnings of pregnancy, implantation describes the moment when the embryo is implanted in the uterine lining. What is at stake during these key few days? Decryption.

Implantation describes the moment when the embryo completely penetrates the lining of the uterus. This key stage of pregnancy takes place from the 7th day after fertilization, that is to say on the 21st or 22nd day after the last menstruation, and until the 10th day of pregnancy.

We often make the shortcut between fertilization and gestation. However, the first days (even the first hours) of pregnancy are rich in events essential for the proper implantation of the embryo in the uterine lining. These main steps:

• The segmentation : after having divided into 2 equal cells (the bastomers) after only 30 hours, the egg continues to divide into 4, 8, then 16 cells, 60 hours after fertilization. We then speak of morula, in comparison to a blackberry.
• the migration : From the 72nd hour, the embryo begins to move from the outer part of the tube towards the uterine cavity. During this 3-day “journey”, the cells that compose it differentiate with on one side, the embryonic button, on the other, the trophoblast, which will become the placenta. This is the blastocyst.
• Arrival in the womb is done 4 to 5 days after fertilization. The embryonic pole then attaches itself to the uterine lining… Implantation can begin.

 Between 7 and 10 days after fertilization, the embryo makes its nest in the uterine lining, again in several stages. On the 7th day of pregnancy, its outer cells (the trophoblast) produce enzymes that allow it to gradually creep into the outer layer of the mucous membrane: the epithelium. This “docking” period lasts until the 9th day after fertilization when the blastocyst completely penetrates the mucosa. Finally, on the 10th day, a temporary plug of uterine fibers plugs the breach created by the passage of the embyron during implantation.

During these few days of implantation, cell development continues to be in full swing:

• during the second week of pregnancy, the embryonic disc, derived from the embryonic button and blastocyst, is formed. First made up of two parts (the sheets), it evolves into 3 sheets during the 5th SA. An essential step because it is from this disc that all the cells of the embryo will be formed.
• The 8th day after fertilization, the amniotic cavity appears when the embryonic button and the trophoblast move away. Surrounded by a membrane, the amnion, the famous bag of water in which the baby develops for 9 months then begins to grow.

While we cannot really talk about implantation symptoms, the hormonal impregnation essential for successful gestation can be the cause of the famous signs of pregnancy that we often try to identify as early as possible (swelling breasts, fatigue, irritability, etc.).

Indeed, during these first days of gestation and implantation, 2 major hormones interact:

• progesterone, produced in large quantities by the corpus luteum in the ovaries (which also diffuses estrogen), prevents normal uterine contractions during menstruation from taking place and contributes to the thickening and vascularization of the uterine lining. Her goal: to maintain the pregnancy.
• l‘hormone HCG (chorionic gonadotropin) is secreted by the trophoblast to maintain the activity of the corpus luteum for 3 months, the time that the placenta can ensure exchanges between mother and child. The secretion of this hormone begins with implantation, 9 days after fertilization. Thus, before becoming pregnant, the level of HCG is stable around 8 IU / l. After implantation, it doubles every 2 to 3 days until about 8 weeks before stabilizing around 5000 IU / L for the remainder of the pregnancy. It is by measuring the level of HCG that the pregnancy is confirmed (by urine test and / or serology), even if the exact dating of the gestation is only possible by ultrasound.

In France, nearly 2% of pregnancies are ectopic (GEU). Primarily attributable to smoking or a history of genital infections, EPs result from implantation and development of the embryo outside the uterus. In the vast majority of cases, it develops in the tube, its passage to the uterine cavity having been blocked by an obstacle. This is called a tubal pregnancy.

A first-order gynecological emergency, ectopic pregnancy is the leading cause of maternal mortality in the first trimester. The explanation: by growing in the tube for 2 to 4 weeks, the embryo ends up breaking it, if no prior treatment has been carried out. Internal bleeding can then take place, hence the need to consult as soon as possible in the event of severe pelvic pain in early pregnancy.

 After plasma HCG testing and ultrasound, management, if necessary, is assessed based on the condition of the tube. It can consist of:

• a salpingotomy, an opening of the fallopian tube under laparoscopy to remove the egg;
• a salpingectomy, removal of the tube if it is very damaged,
• a medical treatment based on methotrexate (one intramuscular dose) to absorb the egg.

Among ectopic pregnancies, gestation is said to be non-tubal when implantation is neither in the uterine lining nor in a tube. In these fortunately extremely rare cases, implantation may have taken place:

• in an ovary (ovarian pregnancy),
• below the internal opening of the cervix (cervical pregnancy),
• in the abdominal cavity (abdominal pregnancy).

Like tubal pregnancies, these GEUs require emergency care, by surgery or medicine.