The length of your menstrual cycle is determined by the number of days from the first day of bleeding to the start of the next period. The length of your cycle—while not on any form of birth control—can be a key indicator of hormonal imbalances and whether or not ovulation is occurring in a regular manner. If ovulation does not occur, pregnancy is not possible.
Normal menstrual cycle A normal menstrual cycle lasts from 21 to 35 days. If you have a normal menstrual cycle, regular ovulation should be occurring and all hormones should be in balance, making natural conception possible.
Short menstrual cycle A short menstrual cycle is anything shorter than 21 days. Shortened cycles can be an indication that the ovaries contain fewer eggs and that menopause may be approaching. As the number of eggs in the ovaries decrease, the brain releases more follicle-stimulating hormone (FSH) to stimulate the ovaries to develop a follicle. This results in earlier development of the follicle and earlier ovulation, creating shortened cycles. This can make conception more difficult.
Long or irregular menstrual cycle A long menstrual cycle lasts more than 35 days. Longer cycles are an indicator that ovulation is not occurring or at least not occurring in a regular manner. This lack of ovulation is known as anovulation. There are many potential causes of anovulation, such as:
Irregularities with the thyroid gland Elevations of the hormone prolactin that can disrupt the brain’s ability to communicate with the ovary Polycystic ovary syndrome (PCOS)
These hormonal imbalances lead to problems with ovulation and fertility, but they may also increase the risk of miscarriage.