Horse that is overdue to foal

Question
I have an 8 year old mare that was due to have a foal on March 9th.  This is her second baby.  She is definitely hanging low, but I have seen no signs of waxing yet.  How long should I wait before calling a vet in?  Also, are there any other obvious signs as to when she is ready to foal?

Answer
Hey Tracy,
  Going overdue up to 4 weeks is normal for some horses.  I had one mare that would always go over at least 3 weeks.  I would call your vet anyways though, because if this isn't normal for her, there could be problems.  Although, by the time you get this email she might already have had it!  Wish you lots of luck...sorry i took so long to reply, i've been away.