39 weeks is the earliest allowed, unless it's medically necessary earlier
Full term is now considered 39 weeks, so unless you want complications I would wait until then. Of course you may have medical reasons to have to have the baby sooner... But normal/low risk is 39 weeks.
Doctors won't induce until 39 weeks, but if you go into labor at 37 weeks it's acceptable and they won't try to stop it.