Every one is different. Timing of the birth/spot the cow picks/some very smart.  Record is 9 calves taken away from moms in like 12 hrs and 3 days to graft 8 of the 9 back. Several not to real moms and one did not even know who his mother was. Do my best to forget about that late March snowstorm over mud years ago.  This is why they are not born all on the same day at this place. It is a tuff call but we had lots of births yesterday 48 hrs before the storm.  Always figure dry and cold not the end of the world but wet and cold is trouble.  Takes every trick in the bag and then some. Good Luck.  Extra hay soon as the storm stops.