Dear Fadila, If your menstrual cycle periodicity has become irregular, it is indeed a difficult task to calculate the day of ovulation by calendar method. Usually, the ovulation occurs 14 days prior to the next expected menses. Now, since you cannot predict the day of your next menses, you cannot calculate the day of ovulation by this formula. However, you need not be disheartened by this. There are few more tools at your disposal which can help you locating the day of ovulation. 1. Ovulation monitoring kit: By using this kit, you can identify the day of ovulation. 2. Ultrasonography: This gives you direct visual evidence of ovulation. It also shows you thickness of the uterine lining (endometrium). Endometrial thickness of minimum 10 mm is conducive for implantation of fertilize ovum (blastocyst). The follicle which ruptures at around its size between 18-21 mm gives best results of fertilization and continuation of pregnancy. These tests should be taken from day 9 of the menstrual cycle. Once you know the the day of ovulation and the size of the follicle, you may plan your intercourse on and around the ovulation. However, if these tests reveal that there is no ovulation, you will have to report to the Gynaecologist and get the treatment for induction of ovulation. I hope this gives you required guidance. Dr. Nishikant