In developing countries aka (LEDCs):
Lack/ no knowledge of family planning, e.g use of contraceptives, pills etc
*High infant mortality rates may cause more births to replace the dead children
*More children for labour
*In some countries, the boy child is highly valued, thus people will keep on giving birth until a male child is conceived
*More children to take care of the parents when old
*Sign of power having many children
In MEDCs birth rates are actually low, so I can't think of any causes in developed countries