Average Menstrual Cycle Length

menstruation_cycleAverage menstrual cycle lengths:

A “menstrual cycle” is the period between the first day of one’s menstrual period to the first day of the next.

The normal menstrual cycle for adolescents is between 21 and 45 days. This is slightly longer and more varied than the normal menstrual cycle for adults.

The normal menstrual cycle for adults is between 21 to 35 days (with 28 days being the average).

Variability of menstrual cycle lengths: 

The length of a woman’s menstrual cycle usually varies somewhat, with some shorter cycles and some longer cycles. A woman who experiences variations of up to eight days between her longest cycles and shortest cycles is considered to have regular menstrual cycles. It is unusual to have cycles that vary by less than four days.

The variability of menstrual cycle lengths is highest for women under 25 years of age and is lowest, that is, most regular, for ages 35 to 39. Subsequently, the variability increases slightly for women aged 40 to 44. Length variation between eight and 20 days in a woman is considered as moderately irregular (and variation of 21 days or more is considered very irregular).

A typical menstrual cycle consists of menstruation (bleeding) for the first 2-7 days, followed by the follicular (pre-ovulation) phrase for 10-16 days, followed by ovulation, followed by the luteal (post-ovulation) phrase for 14 days.

For an individual woman, the follicular (pre-ovulation) phase of her cycle often varies in length from cycle to cycle; by contrast, the length of her luteal (post-ovulation) phase will be fairly consistent from cycle to cycle (14 days is normal).

95% of individuals have follicular phases between 10.3 and 16.3 days. The follicular phase also seems to get significantly shorter with age (14.2 days in women aged 18–24 vs. 10.4 days in women aged 40–44).

What might a shorter menstrual cycle (<21 days) indicate? 

If a menstrual cycle is less than 21 days then it is likely ovulation has not occurred. Shortened cycles can be an indication that the ovaries contain fewer eggs and that menopause may be approaching. As a woman grows older, her menstrual cycle shortens. As the number of eggs available in the ovary 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 and therefore shortened cycles. In addition, sometimes bleeding can occur even when ovulation does not occur, and this may appear as shortened cycles.

What might a longer menstrual cycle (>35 days) indicate?

Longer cycles are an indicator that ovulation is not occurring or at least not in a regular manner which can make conception difficult. Longer cycles are caused by a lack of regular ovulation. During a normal cycle, it is the fall of progesterone that brings upon bleeding. If a follicle does not mature and ovulate, progesterone is never released and the lining of the uterus continues to build in response to estrogen. Eventually, the lining gets so thick that it becomes unstable and like a tower of blocks, eventually falls and bleeding occurs. This bleeding can be unpredictable, and oftentimes very heavy and lasting a prolonged period of time.

What if a person’s menstrual cycles stop occurring? 

When a woman does not have a period, this can be caused by a failure to ovulate. It is common in women who are underweight (the body requires a certain level of body fat for reproduction and menstrual cycles to occur).

If a woman has never had menstrual bleeding, there may have been a problem with the normal development of the uterus or the vagina.

If a woman had menstrual cycles previously, but then stopped, this could be due to a problem with the uterus itself, like scar tissue inside the cavity, or may be due to premature menopause. If the uterus has not formed or if menopause has occurred, pregnancy is not possible. If the absence of menses is due to scar tissue inside the uterus, then this scar tissue will need to be removed as it can interfere with implantation.

Menstrual Cycle Length and IVF Fertility Treatments:

As measured on women undergoing in vitro fertilization, a longer menstrual cycle length is associated with higher pregnancy and delivery rates, even after age adjustment. Delivery rates after IVF have been estimated to be almost doubled for women with a menstrual cycle length of more than 34 days compared with women with a menstrual cycle length shorter than 26 days. A longer menstrual cycle length is also significantly associated with better ovarian response to gonadotropin stimulation and embryo quality.



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