While a menstrual period is generally considered part of a regular menstrual cycle, it's actually not necessary for conception to occur. For a woman to become pregnant, she must ovulate and ovulation can occur with or without a regular period.
It's not unusual for women to ovulate without having regular periods; this can happen for any number of reasons but is common in women who are breastfeeding, approaching menopause or have low body weight, says the American Pregnancy Association.
Irregular periods can make it more difficult to get pregnant if you're trying to conceive by tracking your cycle and predicting ovulation, but irregular menstruation does not necessarily indicate irregular or absent ovulation.
Ovulation generally occurs about 12 to 16 days before a menstrual period, so it's possible for a woman to go without a period for many months, have intercourse around the time of ovulation and become pregnant.
If you're sexually active but experience irregular periods, take regular pregnancy tests since you cannot rely on a missed period as a sign of pregnancy.
Irregular periods are common and may not be a problem, but they can be an indication of a hormonal imbalance, thyroid abnormality or other condition so see your doctor to rule out any issues.