Welcome to HCM.
For irregular periods
predicting fertile period is difficult if not impossible. Only reliable method is follicular growth monitoring on ultrasound
scan from 7th day onwards.Basal body temperature
(BBT) measurement daily in the morning before getting up and charting it out to look for 0.5 Degree Centigrade drop in temperature (ovulation day i.e. most fertile day) is another method.
Wish you all the best.