When you find a partener for you're tamagotchi, you're tamagotchi has a baby(if it's a girl, if it is a boy then it gets one of the baby's his wife had)and at midnight, the parent leaves and you must take care of the baby. The baby is the next generation
ex. You had gen.1 tama, and you're tama parent has a baby, then the parent dies, then the baby is gen.2.