Train travel during final stages of pregnancy

Are there any restrictions for train travel during the final month of pregnancy? My wife is due on Aug. 25th, and we want to travel by train from Kunming to Chengdu on Aug. 8th, as we are planning to have the baby in Chengdu. Will there be any documents or doctor's notes required of us (other than passports) when we buy the train tickets/board the train?