If a sperm does penetrate the egg in the fallopian tube, the egg quickly finishes the maturation process, combines with the sperm, and starts dividing rapidly. It continues on to the uterus, and sometime between 6 and 12 days after ovulation it will implant into the thickened lining in the wall of the uterus and (according to the legal and medical definition) pregnancy officially begins. Note that the calculation of gestation time begins with the date of the first day of your last menstrual period.