Odds of being pregnant with irregular periods?

Hard to say: A young healthy woman has approximately a 20% chance of getting pregnant if she has unprotected sex around the time of ovulation on any given cycle. If you have irregular periods you are likely not ovualting regularly. If you want to be pregnant, medication (possibly clomid) may help. If you don't want to be pregnant, you should still use some contraception.

Still possible: Irregular periods are caused when your body's signals for ovulation and menstruation get confused by hormonal imbalances that can be related to your thyroid (TSH), pituitary (LH/FSH/prolactin), ovaries (estrogen/progesterone). Many external factors can affect the hormones: malnutrition, stress, obesity, for example. Hormonal contraception can help to regulate your periods, which is healthier.