All you need to know about embryo implantation
Embryo implantation is the crucial stage in a natural pregnancy, or in an ART pregnancy, which enables the embryo to continue its development in the mother’s womb.
The phases of the menstrual cycle
The menstrual phase, manifested by menstruation, is the stage of endometrial renewal. The superficial part of the endometrium is evacuated if there has been no pregnancy, in preparation for the next cycle. The menstrual phase is triggered by the fall in progesterone (produced by the ovarian corpus luteum), which signals that fertilization has not taken place, triggering the onset of menstruation, after which the endometrium rebuilds itself to receive a potential embryo during the next cycle
For the endometrium to be ready to receive an embryo, it must be reconstructed for future implantation. The estrogens produced by the ovaries stimulate the growth of the endometrium, and this phase lasts around 14 days (although it can vary from one woman to another). The endometrium thickens on ultrasound. This proliferative phase continues until ovulation.
The secretory phase begins after ovulation. The nature of the endometrium changes: it no longer thickens, but prepares for implantation by becoming more vascularized and loaded with sugar. A complete change takes place in the type of immune cells that will populate the uterus. The endometrium prepares for potential implantation with every cycle!
“There is an implantation window in the middle secretory phase, around one week after ovulation. This window lasts around 4 days: it’s the phase when the endometrium awaits the embryo. An embryo can only implant during this period, which we therefore call the uterine receptivity phase.”
Dr Nathalie Ledee
MD in Obstetrics and Gynecology and PhD in reproductive immunology
The reproductive immunology approach, using the MatriceLab test, offers a better understanding of uterine receptivity, making it possible to propose therapeutic solutions that optimize receptivity and improve birth rates.
Stages of embryo implantation
Embryonic implantation is the process by which the embryo attaches itself to the uterus. It is the most complicated and, still today, the most mysterious stage in the human reproductive process.
The embryo reaches the uterus when it is approximately 5 days old. It must then appose, adhere, and invade the endometrium. From that point on, it will be nourished by the endometrium, which has prepared itself to receive it. This apposition-adhesion-invasion process is estimated to last around 4 days. Once these stages are complete, invasion continues for 3 months, this is the stage of implantation and placental formation.
Implantation failures
Implantation failure corresponds to the absence of attachment despite the fact that the embryo has arrived in the uterus, either spontaneously or after an embryo transfer.
This implantation stage is a “black box” stage, as it cannot be observed in detail. The embryo may have failed to implant:
- because it did not adhere to the mucosa
- because it failed to invade the uterine mucosa
- because the embryo stopped developing.
It is estimated that more than half of all embryos fail at this stage.
Factors influencing embryo implantation
There are many factors influencing embryo implantation.
They may be linked to poor embryonic quality, or to poor preparation of the endometrium itself.
Embryonic quality is greatly affected by maternal age. Indeed, until it hatches to implant, the embryo develops in the oocyte, which provides it with all its nutrition and energy, in addition to the genetic capital of the father and mother, who have merged.
An embryo is always genetically different from its mother (due to the father’s genes), so implantation is a kind of graft or symbiosis.
For the embryo to implant, the endometrium must be sufficiently thick (over 7 mm) and prepared not only to tolerate the embryo, but also to nourish and grow it. A unique immunological reaction takes place during this period to enable the embryo to be welcomed.
For the embryo to adhere, the uterus, which was covered with an anti-adhesive layer, must become adhesive. Then, upon invasion, the endometrium must recognize the embryo as an entity of its own and begin to nourish and protect it. If this reaction doesn’t work, at any stage in the process, the embryo will either fail to implant or fail to undergo implantation (miscarriage).
Complications of embryo implantation
The most frequent complication of implantation occurs when the embryo does not implant in the uterus but elsewhere, most often in the fallopian tubes (which theoretically lead the embryo into the uterus). This is known as an ectopic pregnancy. The embryo develops, but its invasion is not controlled by the endometrium. It then penetrates the fallopian tube to the blood vessels, which can cause hemorrhage.
Another potential complication is multiple pregnancy. Several embryos are implanted at the same time, most often following multiple ovulation (after ovarian stimulation) which has generated several embryos. With in vitro fertilization, the number of embryos replaced is controlled, which is not the case with unmonitored ovarian stimulation.
A miscarriage represents a successful implantation but a failure of the embryo to fully embed itself in the endometrium. There are also many causes of miscarriage (uterine, genetic, autoimmune or immunological).
MatriceLab helps improve the embryo implantation process
MatriceLab offers an immunological test on the endometrium when it is theoretically ready to receive an embryo. The aim: to document whether its condition will allow implantation. We assess whether the endometrium is ready for adhesion, and whether the immunological reaction that must occur is adapted to allow invasion without rejection.
This test is of particular interest in cases of failed implantation or repeated unexplained miscarriage. Our aim is to identify the precise stage that poses a problem, so that we can tailor care to ensure that the next attempt is a success.