Period schedules

Just a question to all my fellow ladies: do most women have such regular periods that if you are just one day late, you could be pregnant or something could be wrong? Is it just that a lot of women have stabilized to an exact "time of the month"? I'm about to turn 16 and I almost never have a period on the day it was the month before. Is this because I may just have an irregular period? I mean, I'm still a virgin and have never engaged in sexual activities, but I still want to know, for later reference! :) I'd love the feedback!