i am very confused about something and i was wondering could u help me. i am pregnant and i had sex on the 9th and 10th of november 2012 and my cycle came on on the 14th and went off on he 20th of the same month. my next cycle came in december on the 14th and went off on the 18th. the next time i had sex was on christmas night. a pregnancy is 40 weeks right? well i went to my due date and counted back 40 weeks and the date i got was the 15th of december 2012. im 16 weeks and some days. my due date is on september 20, 2013. ok the question is, i dont know how im pregnant and i didnt have sex for a while and the date i counted up was a day that my cycle was on. please help me.