ILoveme gave a good list. A couple others that I see a lot that might be helpful:
CP - Chemical Pregnancy OR Cervical Position (context will tell you which)
TWW - Two Week Wait (another way to write it), which is the time from Ovulation to your period
AF - Aunt Flo, AKA: period. Sometimes described as
FX - Fingers Crossed
AFM - As for me
If others pop up that you don't know, just ask. Most of the ladies here are super helpful!