41W4D. He was 7# 4oz, so certainly not too big. I'm 100% certain of my LMP & fairly sure I ovulated around day 14 of that cycle (although that
I'm not as certain of. I'd only had 3 cycles after getting off the pill before conceiving & I hadn't been able to detect ovulation based on basal temp.)
As a previous poster pointed out, 40W isn't really average. It is "Naegel's rule" - named after a German OB in the 19th century who just decided gestation was 10 moons - 40 weeks. It was never based on empirical evidence!
So sad how many components of modern American maternity care are NOT evidence-based.)
(I believe based on meta-analyses) by a doc named Mittendorf shows that 41W1D is average for first-time Moms who are white. I believe a few of the other ethnic groups for which he collected data were a little bit shorter than that - but still over 40W.