Improved birth control implies "prevents unwanted births". Some places without 'modern' birth control have low rates of unwanted pregnancy, but some places are

Improved birth control implies "prevents unwanted births". Some places without 'modern' birth control have low rates of unwanted pregnancy, but some places are pretty high. They both lower birth rates, but better birth control will only drop rates to "as many kids as people want to have", which is >0 for a lot of people. Voluntarily childless has a personal/psychological element - it's about not wanting a family in the first place - and can take personal birth rates down to 0.