If you ovulated and the egg met a sperm, amazing things will happen fast. It takes just three days from fertilization for a single egg to divide into a ball of 58 cells. By the end of the week, this ball, called the blastocyst, will have reached the uterus, where it will start to implant in the lining. It will be a couple of weeks before you know whether you've conceived, but special hormones kick in now to help maintain the pregnancy.