What is progesterone timing, and why is it recommended?

Progesterone timing is a very complex thing, but to boil it down, the premise is that you pull blood every other day about when she comes into heat and can track when she ovulates and when she's going to be most fertile. So using that to guide breeding has been shown to increase pregnancy rates, increase litter sizes, and give you an accurate due date, so you know what day to be on the watch.