I would say quite a lot, actually:
- You've now got the responsibility of a child, bang goes your young, carefree days
- How are you supposed to support a child at that age?
- You were planning on going to University? That's probably sunk unless you get someone to look after the baby (which they shouldn't have to)
- It's probably a bit weird celebrating your child's 21st birthday before your own 40th
- Is your relationship solid? Sucks if it isn't, especially now you have the baby
- Can you afford somewhere to live that's half decent?
The list goes on. Needless to say it's not a good idea.