I think it would be. I believe it takes a week or two to actually conceive, so for instance...
If a woman's period is due monday and she has sex sunday and gets pregnant, she'd probably still have her period tuesday because conception would take longer then 24 hours to happen. But then she might not have her period the next month.
Your best bet if you think you're pregnant is to take a test, not to go by your menstrual cycle.