Well, chances are less, but not impossible. Yes, irregular periods decrease your chance of becoming pregnant. Many a times there is no ovulation when there are irregular periods. Also it is very difficult to know the exact date of ovulation when your periods are irregular. So calculating the fertile time becomes difficult. Some irregular periods are due to hormone imbalance. So this causes difficulty in the fertilized ovum to implant in the uterus.
Hence it is important to find the cause of irregular periods and treat them if you want to conceive.
Wait until the maximum number of days after which you have had a period in the past. If you still do not get them, take a pregnancy test, preferably a blood HCG estimation as this is more reliable. If this is negative, it is best to consult a gynecologist and find out why you have irregular periods. High level of stress, dramatic changes in your life style, high cholesterol, hormonal imbalance as seen in thyroid problems and PCOS, chain smoking, may be some of the common causes of your irregular periods. Cushing’s disease and disorders of adrenal gland, tumors of the ovaries and adrenals, insulin resistance and obesity are some of the other factors. Prolonged use of oral contraceptive pills is another cause. Other important local causes of irregular periods are PCOS and fibroid uterus. PCOS causes cysts to form on the ovaries, interfering with regular ovulation. Diagnosis is by blood tests like blood insulin levels, serum testosterone and androgen levels and by ultrasound. I sincerely advice you to consult a gynecologist and get it evaluated.