Originally Posted by
atomic sagebrush
First day of actual flow PROVIDED that you don't always just get your period with a couple days of light spotting at the start.
By this I mean, if you ovulate and then have a 14 day LP and then a light day and then a heavy day, and that is just the pattern your period takes, then your light day is obvs. CD 1.
But if you normally have AF come on all at once and heavy from the start, and then all of a sudden you start having 2 days of light spotting 12/13 DPO and then red flow starts 14 DPO and you have a normal period after, then the spotting is at the end of the LP in that case and isn't your period till the red flow stars.
So to sum up - usually the first day of red flow unless your period comes in a particualr distinct pattern that always starts with spotting