If you have a short menstrual cycle, or you don't ovulate mid cycle, this could mean that you didn't actually conceive until days before your period was to start. For example, if a woman is regularly maintaining a 26 day cycle, chances are she'll ovulate around day 13. If she had unprotected sex on that day, and sperm found egg on that day, it would still take the fertilized egg six to twelve days to reach and implant in the uterine lining. Following this time line, her body wouldn't know to begin producing the hormones that cease menstruation until somewhere between day 20 to 25 of her cycle. That could leave as little as one day between conception and menstruation. If those hormone levels are insufficient, she could have a lighter than usual period, or even one that seems entirely normal and wouldn't miss a period until she was already around six weeks pregnant. It should be noted that even in this case, ovulation would not occur again following that period.
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