The Dong (侗) people is one of the 56 official ethnic groups of China. One of Dong tribe that is staying in a village Guizhou have a special way of contraceptive, each family pledge to conceives only 2 Children in their life time. Then, there is a secret remedy that pastes only from mother to daughter, which claims to be able to change the sex of the baby while they are still inside the womb. Secret revealed only before death. (Link)
For 50 years or so every family in the village have had two children each, one boy and one girl, only.