When a sperm joins with an egg, the two form a single cell called a zygote that contains DNA from both parents. This is fertilization, also called conception. Within about 3 days, the zygote begins dividing into more cells. By about 5-6 days, it has traveled through the fallopian tube into the uterus. At this stage, it's called a blastocyst. It attaches, or implants, to the uterus walls. By day 10-12, the zygote has become an embryo.