Pregnancy can only occur when sperm enters the va*gina and fertilizes an egg, a process known as conception. Once the sperm has successfully made its way through the cervix into the uterus and found its target – an egg located in one of the Fallopian tubes - it binds to the egg’s outer membrane and penetrates it, releasing its genetic material. If a female's ovum is already present in her Fallopian tube then if two sperms penetrate that same ovum at once, two separate sets of chromosomes are combined resulting in a fraternal twin pregnancy. On average, after successful fertilization takes place, it takes about six to ten days before implantation occurs within your uterine lining which marks official pregnancy start date.