After the sperm reaches the fallopian tubes, it meets the egg, destroys its protein membrane and penetrates inside. This is how fertilization takes place, which becomes the beginning of the embryo's developmental path. After the merger of the father and mother materials, a fetal egg is formed - the unborn child. A fetal egg goes through certain stages of its development, which are characterized by quantitative and qualitative changes. But in order for the further development of the fetal egg to become possible, he needs a nutrient medium from which nutrients will come over the next nine months. The uterus becomes such an environment.
So, the fetal egg in the fallopian tubes descends directly into the uterus. This is due to the presence of microscopic villi inside the fallopian tubes, which promote the ovum. If the fallopian tubes have pathologies (adhesions, bends), then implantation of the fetal egg is much more complicated, since the path to the uterus becomes problematic. In this case, the fetal egg may linger in the fallopian tubes and attach there. This phenomenon is called ectopic (tubal) pregnancy and requires medical intervention.
Usually, with the normal structure of the fallopian tubes, the ovum enters the uterus within four days. At this time, it is in the morula stage, and when it enters the uterus, it is in the blastocyst stage. This stage is characterized by the fact that the fetal egg is ready for new changes. The blastomeres begin to form an embryoblast - the part from which the embryo will further develop, and the second part of the cells - trophoblast - from which the nutrient membrane for the egg will form (the most productive part of the trophoblast will later turn into a placenta).
At this stage, implantation of the ovum is just beginning. Under the influence of hormonal changes, the uterus also prepares for it. After the follicle burst, a corpus luteum formed, releasing largely progesterone and, to a lesser extent, estrogen. It is progesterone that signals the uterine mucosa about the need to prepare its layer. First of all, the quality of the endometrium in the uterus changes - it is called the decidual membrane during pregnancy. What is the difference between the new endometrium? The main feature of this endometrium is the accumulation on its thickness of biologically active compounds that nourish the embryo. These are glycogen, mucopolysaccharides, lipids, salts and trace elements, enzymes, immunoglobulins - all of them are simply necessary for implantation of the fetal egg to continue the full development of the fetus. In other cases, when there are defects in the endometrium and its functions, women cannot become pregnant for a long time, and sometimes even become infertile.
With normal endometrium, implantation of the fetal egg lasts about two days. During this time, blood vessels dilate at the site of attachment (nidation), the endometrium thickens. Such changes indicate that the uterus is ready to accept a blastocyst. When the fetal egg is immersed in the thickness of the endometrium, it is as if enveloped by the fibers of the uterus. Sometimes such an attachment may be accompanied by minor bleeding, which can be mistaken for untimely onset of menstruation. However, the amount of blood is extremely small - a few drops or a small spot, after which such "bleeding" stops. The implantation of a fetal egg, the signs of which are not felt by a woman, only in this way makes itself felt, since this happens in the middle of the cycle, after ovulation, and is still far from the onset of menstruation. If women monitor their cycle and know the time of ovulation, then with a high degree of probability they can hope for pregnancy. Only in a small number of women implantation of the fetal egg, sensations during implantation in any way manifest. Sometimes tingling, nausea, a feeling of heaviness are felt. However, they are caused not by implantation itself, but by hormonal changes that occur in the body.