What factors contributes to keep up birth number? (Kebir Blue)

I have noticed that my birth nauber has been dropping since I started from around 840,000 per yr down to 780,000 per yr now.

Meanwhile my death number has gone up from 300,000 per yr to almost 550,000 per yr.

My population has grown from initial 6 million to almost 49 million now.

Why does the birth number drop with a population that is 8 times higher?

If I had a birth number of 840,000 for a 6 million population shouldn't birth number be much higher with a much larger population?

So what is the factors that one can do to make birth number go up?

The game automatically constrains birth rates, and increases death rates, in high population countries as a check against accumulating huge amounts of pop.

the only thing you can do is build more hospitals. your health index has to rise the higher your pop gets, especially once it goes past around 60m, 70m, 80m. health index needs to be around high 170 to 200+ to keep up with that.