When an egg and a sperm meet, this process is known as fertilization. The zygote will begin the process of dividing into cells, eventually becoming a baby. When the process of dividing the zygote doesn't work, "sometimes the result is twins and multiples." There are identical twins and fraternal twins. When an egg is fertilized, the babies will be identical. However, when two eggs are fertilized, the babies will be fraternal. When the cells do not divide the twins will be conjoined. (Ciccarelli & White, 2015). Prenatal development involves three different stages during pregnancy. The first period is known as the germinal period. This period lasts two weeks during which the zygote moves towards the uterus, the mass of cells attaches to the walls of the uterus, the placenta begins to develop and the umbilical cord begins to develop. During this period the cells begin to develop into the various cells that make up the human body. (Ciccarelli & Bianco,