How does the Population refer to how delveopletd a country is?

So. If the country was doing horrible, and the health system was basically non existent. Then the death rate and the birth rate would relatively stay the same but be very high. Lots of babies could be born but then a lot of them could die. Like the 1600s and before that for UK. If the country however had an amazing health system and overall a great HDI. Then the death rate and birth rate would be low. This is because women tend to have more education so have less children and have them later in life. And also because common cuts cant really kill people back in the 1600s. An example would be Japan or Germany.