juniorpc is right, you really have to keep an eye on them when they get close to the due date. In fact, that post reminded me of something.

Start taking her temperature twice a day, 1 to 1-1/2 weeks before her due date and write it down. When you see a temperature drop of about 2 degrees below the average, she is within 24 hours of delivering. It is possible to miss the drop, but we have only missed it once in all the litters we have had.

Bruce