Labs first heat cycle

Question
I have a pair of Labrador females from the same litter that just turned 7 months.  They're scheduled to be spayed in 3 weeks.  What are the chances they will begin their first heat cycle before this?

Answer
That is hard to say.  Usually they pick the worst possible time.  I would guess the odds are about even.  If they start into heat, likely the vet will make you wait until they are done.  

You will be stuck with the hassle, mess, and risks. The second and third weeks are when the most risk of accidental breeding is. Don't take this lightly. They will do anything to get loose and get to a male. Once loose, an accidental breeding and an unwanted litter is far from the worst that can happen. Such breedings can also result in fatal infections. One can get killed when she dashes heedlessly across the road in front of a car. Other dogs or coyotes may kill her.