You may not be ovulating every cycle which could cause you to skip periods. I'm diagnosed with PCOS and only have my period every few months and rarely ovulate. It's extremely irregular and I have no way of predicting it what-so-ever. It's especially a pain because even if I were to become pregnant, I wouldn't realize it right away since I don't know when I'd be considered 'late' for AF.
You should visit your doctor to inquire about your irregular periods. They may suggest that if you go more than 3 months without one, to attempt to induce one by taking progesterone. Each time you miss a period the lining becomes thicker and thicker as it isn't shed and able to be broken down as it would be during your period. Think of it as spring cleaning lol.
possibly if you two are not having sex when you are ovulating