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The concept is that ovulation always occurs 14 days prior to your next period no matter if it is regular or irregular. When there are irregular cycles it will be difficult to predict. First you need to regularise your cycles by getting birth control pills frem your Gynaecologist. Once it regularises you can get pregnant. Better to go for ultrasound
ovulation study to know when the follicle is ruptured and ovulation occurred and that day you can have sex.