With ejaculation, millions of sperm are released into the vagina and fight their way up the female genital tract to reach the fallopian tubes. This is where the egg is found once ovulation has occurred, as the fallopian tubes are responsible for collecting it. However, only a few sperm will reach the fallopian tubes and, finally, only one will be able to fertilize the egg.
Thus, although the pregnancy develops in the uterus, the meeting of the egg and the sperm takes place in the fallopian tube. Therefore, the embryo must descend to implant in the uterine endometrium and thus begin gestation.