When a single sperm enters the mother’s egg, a genetically distict human being comes into existance. This is the zygote stage. The zygote spends the next few days traveling down the fallopian tube.
The zygote continues to divide, creating an inner group of cells within an outer shell. This stage is called a blastocyst. The inner group of cells will become the embryo, while the outer group of cells will become the membranes that nourish and protect it.
The blastocyst reaches the womb (uterus) around day 5, and implants into the uterine wall on about day 6.