We actually aren't having enough babies in developed countries - KyubinoKitsune is right; it's the developing countries that need to start using birth control. Even then, that's not the major problem. Places like Africa have high child mortality rates, so families need more children in case some die. They also need those kids to do things like help on a farm or whatever, so that they can sustain the family.
The solution there is make sure everyone gets food and stuff.