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Is my due date wrong or is my conception date wrong? - due date calendar conception date

I think I was pregnant March 23, 2009, but my deadline is the end of January 25th round I counted up to 38 weeks and the deadline was December 14, 2009. Then he told me that 40 weeks, I December 28, 2009 ended. 25. January is only 44 and a half weeks after 23 March. I even had the dates and the maturity of the 14th December. I'm at that point, I thought, or the doctor put the date May to 44.5 weeks after thinking wrong? Is it possible that the child may later develop in the uterus and extend my time for more than a month? I am very confused. Please help.