The doctor, a silly question, but is very important how to define or determine 1 day of a cycle, if in...

[Dasha]

The doctor, a silly question, but is very important how to define or determine 1 day of a cycle if in 3 one o'clock in the morning the menses has begun? To start with number which was or has come or stepped?

[Malanova T.B.]

Since that day that has come or stepped.