I have an OvaCue with oral and vaginal sensor. Lightly used, works great. This is the updated version with a color monitor. On the Ova cue web page, it explains how it works. Essentially measuring the electrolytes in the saliva and vaginal fluid, it tells you when you are fertile and confirms ovulation if the vaginal sesor is also used. It works great! It sells for 350 new. I am looking to sell the bundle for $225 plus shipping.