The entire process of pregnancy begins with the fertilization of the woman’s egg by the man’s sperm. When the egg is penetrated by it, this is known as fertilization. At this point, there is no longer sperm and egg, but the product of their union which is known as a zygote appears. The zygote is a one-celled organism, which begins its long journey toward the uterus. This process can take around a week or so. Within about twenty four to thirty six hours after fertilization, many cell divisions take place and the zygote transforms completely. It is called blastocyst and contains two distinct masses of cells, one mass that will become the embryo, and another one that will form the placenta. When the blastocyst eventually makes its way to the uterus, it is ready to attach itself to the lining of the mother’s uterus. On this stage implantation bleeding can be experienced. Read why this happens and how long implantation actually lasts.