Normal: Normal cycle length measured from the start of one period to the start of the next is anywhere from 21 days to 35 or 42 days depending on your source and there is normally a little variation due to the fact that some eggs mature faster than others. This means that periods do not normally start on the same day each month since months vary from 28-31 days. Hope this answers your question.
Answered 4/24/2015
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