Following the ejaculation a large number of sperm cells find themselves in the rear part of the vagina, near the uterine cervix (portio vaginalis uteri). The path that the sperm cells must travel from the portio to their meeting with the oocyte in the ampullary part of the fallopian tube is 13-15 cm long. Along this stretch, sperm cells go through a further maturation process, the so-called capacitation.
The mechanisms that guide the sperm cells to the oocyte have not been researched in all their facets.
In order, though, that a sufficient number of sperm cells appear in the ampulla at the right time, a large number of sperm cells must be present in the ejaculate.
Of the roughly 200 million ejaculated sperm cells only a few hundred are able to traverse the long way through the cervix, the uterus, and past the fallopian tube isthmus to the tube's ampullary region to there meet oocyte.
Along the way whole groups of sperm cells can halt at certain places and enter a phase of reduced activity. That is why a portion of the sperm cells can retain their fertilizing capability for up to 4 days.