I found it funny when talking to my elderly (guessing she is in her 80s) neighbor and she said she was 4 weeks late with her 1st and her second was due in december but came in february. at first i was like "she must not be remembering it correctly. even if she was due at the END of dec and had the baby at the beg of feb, that's still like 6 weeks past the due date!" but then i realized what someone else on here said...they didn't used to have early pregnancy tests. you used to have to wait until you were many weeks pg to confirm that. also, they didn't have ovulation predictor kids, ultrasounds, etc etc. so it was total guesswork about when the baby was conceived and when the due date would be. nowadays of course its all like you are supposed to go on this ONE day, and then you are late. its kind of silly.
i read that 90% of women have their baby between 38-42 weeks. so i would say odds are pretty good that yours will come in that time frame.
with my first i was 9 days late. however, i didn't know at the time (never really paid attention) that i always have 35-36 day cycles and ovulate around cd 20. knowing that now, i think my due date this time is a lot more accurate. if you take the fact that i ovulate 6 days after the traditional "cd 14" ovulation, then she was really only 3 days late. its not an exact science, but OBs schedule inductions as if it were