When is the first day of period ?

So glow said my period would start on the 5th. It was correct however lately I’ve been spotting for 3 days first and won’t get a full flow until day 4. This has been happening since my second miscarriage. Has this happened to anyone? And when is the first day of my period considered? Is it the day of the full flow or the light light spotting where I don’t stain my pad that much?