40 weeks is fine if you're monitored closely (I'd want a BPP including an NST twice a week past 38 weeks).
I went into labor on my own with my last (baby #3) at 39w1d.. had a scary short 2 hour labor with him.. almost didn't make it to the hospital and arrived at 9cm
Not that it's reassuring at all, but I don't personally rate my natural labor (I had no drugs at all with him - not an option that late in the game) as any better/worse than my previous two inductions by AROM with some supplemental pitocin (only for the first half or so of my labors.. once my body took off, they stopped it). If anything, my spontaneous was scarier since it went so quickly... and my inductions were all of 4 and 5 hours, so they weren't exactly lengthy either.
I had no problems delivering any of mine, in fact my last, who was the biggest at 9lbs 5oz was the easiest of the 3 - but that may just have been because he was my third

He was over a pound bigger than my first two though.
My inductions were at 39w wit my first (didn't know any better to ask to go longer, I was SO ready to be done at that point) and then 38w5d with my second. She was actually scheduled for the week after but moved up because my BPP's showed some signs she wasn't doing so great. She was born with TWO knots in her cord, so I'm glad she got out when she did (and did so safely - some cord knots can tighten during labor/delivery, but we didn't know they were there until after she was out), even though I realized the "issues" we had been seeing were more likely related to that than me being diabetic.. no regrets with her
