I believe the answer is: improved healthcare, technology, sanitation, carrying capacity---which leads to people living longer and having more babies
Here are several reasons why fertility rate in an area could go down:
- Increased death rate among young people
- Unsanitary environment which could resulted in various type of diseases
- Problems during child bearing
- not having enough money to sustain family's health.
All of the things that mentioned above address all of these low fertility rates issues
