During ovulation, the egg is released from the follicle on the ovary and travels to the fallopian tube. Under favorable circumstances, there it is fertilized by a spermatozoon - conception occurs. Further, with the help of contractions of the muscular membrane of the fallopian tube and wave-like shocks of the villi on the mucosa, the embryo moves into the uterine cavity. The journey takes three or four days. At this time, pinopodia are formed on the outer cover of the embryo - special structures with which it is attached to the endometrium. This is how normal conception and implantation of the blastocyst into the uterus occurs.
Under adverse circumstances, obstacles are formed on the way of a fertilized egg, due to which it does not reach its destination and attaches itself in places unsuitable for this. Most often this occurs in the fallopian tube from the side of the ovary where the egg matured. Further, the embryo begins to develop, but this happens incompletely, since there are no appropriate conditions in places atypical for pregnancy. Nevertheless, the embryo increases in size, in parallel with this, it destroys the underlying tissues, which can provoke their rupture and intra-abdominal bleeding.