The average menstrual cycle lasts 28 days. But there are women whose cycles are as short as 21 days, and as long as 35, who do not have any fertility problems as a result. Most women have periods that last from three to five days, though they can also last up to a week. If your menstrual periods arrive at the same intervals all the time, regardless of the amount of days your cycle lasts, they are regular menstrual periods.
Having regular menstrual periods during which you ovulate should mean that you are able to conceive within six months to a year if you have intercourse during your fertile period in the absence of other medical factors, of course. But if the post-ovulatory stage of your cycle, the luteal phase, is too short (shorter than 10 days) or too long (longer than 17 days), it could affect your chances of developing a healthy pregnancy.
Hormonal factors can contribute to any fertilized egg being "washed away" by a period in this case. This is called luteal phase defect or LPD. Hormonal birth control usage can affect the length and pattern of a cycle, and when you just come off off the pill, for instance, it can take a while for your cycle to return to normal. But if you did not just stop using the pill, and still have an irregular menstrual cycle, that comes and goes as it pleases, you should consider seeing a doctor to check for any reproductive health conditions such as polycystic ovary syndrome (PCOS).