What I can tell you is our guppies gave birth during daylight hours. Their behavior before hand would differ depending on which guppy as each has their own personality. Basically though, they would hang out at the top of the water first in one of the corners to hide, breathing quite heavily. When close to birth, they changed positions and go to the bottom very close to the gravel. There they pop out a few then do this ritual all over again til the next go round.
As far as the gavid spot being offset, I cant explain that one so maybe someone else can explain it. I have learned when the birth canal discolors(almost brownish) and becomes thin, the gavid spot becomes quite black in color and when their air bubble almost disappears they are going to give birth soon; almost always by the next day. Their stomach looks like they ATE way too much........lol
I dont know if that helped at all, but ............ 8)