I have written about this before here: https://news.ycombinator.com/item?id=44098431

BTW, the fertility rate is _increasing_ now (granted from an existing base of 1.2TFR) in the richer states of india, due to better availability of IVF and in general healthcare.

The combination of lack of prosperity as well as the effect of nuclearisation that I mentioned above was what made it go weirdly low. It's not that low if you exclude unintentionally non-reproducing couples. I'm not saying its replacement rate, but its also not 1.2.

Many poorer states of india will face the same nuclearisation of the family unit, but crucially when healthcare is more generally available, so you won't see those parts go as low as 1.2. Again, replacement rate is almost impossible in a nuclear family unit, unless you manage to substitute something else that contributes the benefits, i.e reinvent joint families from first principles (and maybe it will be better!)