I had a 1 night stand with a woman. She informed me nearly 8 weeks later that she was pregnant. According to a source, her expected due date was October 17th in which she ended up having an emergency C-Section. Is it at all possible to have an Expected Due Date 2.5 weeks later than the average 40 week pregnancy? If so, I am curious to know the percentage of babies born at an EXPECTED 42.5 week date. I do realize there is a very rare possibility that the baby could have miraculously waited 2-3 weeks to be born if the Expected Due Date was at the average 40 week period (Approx Sept 29-Oct 1). Thanks for your help.

Sure it's possible. The only way you can know without a doubt if it's yours though is to get a paternity test done.

Last question, when mothers go into the OB GYN office and are discussing Expected Due Date, what factors would make a baby last 42.5 weeks rather than 40 weeks? Is it based on their menstrual cycle of 28 vs 32 days? Just curious - I plan to get a paternity test but I'd like to know the likelihood going in

Delivery dates and conception dates are estimated, they are basically give or take one to two weeks.

Again, dates and expected delivery charges as she nears delivery. Some babies just are slow to come, and labor or C section not done until the child is too far due, or see issue.

Again, none of the "dates" matter, they are all educated guesses by doctors anyway.

If there is a paternity issue, you have to do DNA test, only way to know

'Is it based on their menstrual cycle of 28 vs 32 days? '

Not at all. Some say that first borns are the most likely to take the longest. There are no ways to know or predict. Keep in mind that 40 weeks is just an average, and the RANGE is quite wide. That's in addition to what has been mentioned - knowing actual date of conception is usually just a guess, unless the woman only had sex once.

Read this one:
http://spacefem.com/pregnant/charts