I believe (in the UK) 24 weeks is the age doctors have to intervene to help the LO (assuming it is believed to be in the best interests). However, a baby born before that is putting up a fight, they would not just walk away.
I don't know if its NHS or hospital policy. But at my hospital from 20 weeks, if you have a pregnancy problem, they will do alot more for you, eg trying to stop pre-term labour. Prior to this they will send you for reassurance scans and check ups etc, but they don't really intervene and tend to let nature run its course.
I am assuming you are a first time mum? I think around the 22-23 week point alot of first time mums worry about this. The 24 week mark seems like a big milestone. I'm on my second and I have found pregnancy alot easier this time round, mainly because chasing after a 12 month old, I don't have time to think about being pregnant.