My 3 spontaneous (aka not induced) labors all started differently so I'm not much help BUT
if you chart BBT, a drop below a sort-of-reverse-cover-line signals impending labor (within 24 hours). Or so I've heard... I only got 2 temps before #2 was born at 38w6d and I hadn't even started charting when #3 came at 36w1d. Vets use this tool to predict when dogs will whelp.