From conception to implantation
The fertilized cell that results from the fusion of the egg and sperm is called a zygote, which divides into two identical cells and continues to divide as it begins its journey down the fallopian tube until it forms a bundle of cells known as a morula. By the time it reaches the uterus, it forms a bundle of around 100 cells, called a blastocyst. About a week after fertilization, the blastocyst embeds itself in the lining of the uterus, the endometrium. At this point the pregnancy is established; the blastocyst develops into an embryo and the placenta develops.
The hormone human chorionic gonadotrophin (hCG) is released; this stimulates the production of progesterone, which maintains the lining of the uterus.