In this period of time, it takes about 11-15 days for the zygote to attach to the uterine wall, causing the zygote to form an attachment. The next stage of prenatal development is the embryonic period. This is the period of prenatal development that occurs two to eight weeks after conception. During this time, cells multiply rapidly, support systems begin to form, and organs begin to appear. During this period, the umbilical cord, the life support system that connects the baby to the placenta, is formed. The placenta allows small blood vessels to pass from mother to baby. This is how the baby can receive food from his mother. Mothers need to be very careful about what they eat and drink, because everything they do will affect their baby. If they drink or take drugs, this can seriously affect the child's development. By the time most women find out they are pregnant (5-8 weeks), the organs have already started to form. "At around 21 days the eyes begin to appear and at 24 days the heart cells begin to differentiate." During the fourth week, your arms and legs will begin to grow. At eight weeks, the organism is about an inch long and the face has begun to develop.