Using GDP per capita as a factor of its world status, would place Thailand around 70th in the world (PPP not nominal). Hardly a developed economy. Now also throw the income distribution as variable in the mix and you could easily come to the conclusion that Thailand is 1st world for the happy few and 3rd world for the majority.
The reason why people have many or few children is correlated with one's household economy. No money, lots of children, lots of money, few children. Thailand is an odd one out here. Thailand's population is poor by definition, but its birthrate is on par with the modern richer countries. Total recipe for disaster in the present political set up. Who will support the older and progressively top of the population pyramid?