My son was born at 29 weeks and 4 days and he was only in the hospital a week and a half.
that would be a rarity for a baby to go home at thirty one weeks gestational age. babies are so unstable respiratory wise they are usually kept until at least 35 weeks gestation.
often, when a baby is born we use the Ballard scale to confirm gestational age. this tells us how mature the baby is and we can date how old the baby is using the scale. Reported maternal dates are becoming so unreliable now-a-days - many will report implantation bleeding (presumably) as the last period, especially with people saying they were "on the pill" (gives a lighter period), often people are not even keeping track of periods, or we've had people flat out lie to us in the office due to the fact they want a certain partner to think he is the baby's father.
often, we can be "off" on our due dates. when my cousins baby was born, there was a guy in there who had a 10 lb baby - in the regular nursery - who said his baby was born at "32" weeks.
Im like... BS. at this hospital, ALL 35 week babies... and below... spend at least the first few hours in the NICU for stablization. Usually only the 35 weekers will even be allowed OUT. i didn't say it to his face of course, but i was like - i am calling your bluff!!