My midwives don't like to let anyone go over 41 weeks due to higher probability of complications. I'm a bit concerned about the placenta failing, or the baby aspirating meconium, so I'm not going to go all the way to 42. I'd be comfortable going to 41w4d max. I'm sure of my conception date, btw. I think it's more fuzzy for people who aren't.
I just wanted to take a second to respond to this. There have been several studies that show that, statistically, the risks of being postterm increase slightly at 42 weeks and moderately at 43 weeks. Before these time points (ie, up until 42 weeks), there is no more statistical risk of complications than if the baby were born at 40 weeks.
That being said, I'll admit that going to 42 weeks makes me nervous, a bit. I'm also quite sure of my dates and so if I do end up going to 42 weeks, that 42 weeks is ACTUALLY 42 weeks, and not, say, 41 weeks or 40 weeks like some women's could be. I told my midwife I absolutely refuse to go past 42 weeks without a biophysical profile to show that the placenta is still working and the baby is still healthy- and no matter what, I won't go past 43 weeks. Period. But that average birth time for first time moms at my midwife practice is 41w3d I believe, so we'll see how this goes!