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Platinum Supermommy
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Join Date: Aug 2006
Location: Alberta, Canada
Posts: 10,165
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Because there can be other reasons for spotting other than the arrival of menstruation. If actual flow hasn't started, then progesterone levels haven't dropped low enough to imply the start of a new cycle. It's also important to know the accurate length of your LP and spotting days count as part of that.
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