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Dr. Jerilynn C. Prior, Scientific Director, Centre for Menstrual Cycle and Ovulation Research

Ask Jerilynn |

What does a "regular cycle" mean?

Q: 
I enjoyed being part of the Menstruation and Ovulation Study and am excited to learn our results. But the menstrual cycle questions got me to thinking. What does it mean to have a regular menstrual cycle?

A: 
Thanks for participating in MOS. We also are excited to see what we have learned. We've just gotten the last urine test results, so we are now starting to analyze all of the information from the 610 women who participated. Keep tuned to the CeMCOR website!

Your question about regular cycles is a very good one.

Let me start to answer by telling a story on myself. Then I'll go on to describe what we used to consider "regular" to mean and some recently proposed standards.

I remember being asked if my cycles were regular on a student health questionnaire for medical school. I said, "Yes." Years later, I recall that answer with chagrin. Part of it was true - I could count on my period coming predictably. However, if "regular cycles" were meant to indicate "normal in length," I didn't answer properly. At that time in my life, as a very stressed 22 year old who had just become one of six women in a very man-centred medical school class, living alone, many miles from my friends and family and barely managing to pay for rent and food, my cycles were too far apart. My periods came only every 36 to 42 days. They came regularly, but they were too far apart to be what we have called normal.

The average menstrual cycle length (cycle length being from the start of one period until the day before the next flow) in many studies, and from many countries, is about 28 days, or a lunar month. However, "regular" has two possible meanings:

  1. How many days long is a normal menstrual cycle (how short is too short, and how long is too long)? and
  2. How variable in length, from one cycle to the next or over a year, is normal?

When I was first starting to study women's reproduction in the 1980s, the answer to the first question was: a cycle is normally between 21 and 36 days long (1). The question about how variable it could be within a normal woman was not provided but was implied that variability should be less than a week. That author also didn't say how he decided what to call normal.

The right way to assess what is normal is to study the average and the variability in a randomly sampled total population. Interestingly, although hundreds of thousands of women have taken part in menstrual cycle studies, there is only one published study that is population-based and asked about cycle lengths and variability over one year (2). This study randomly invited women in the county of Copenhagen in Denmark to complete a mailed questionnaire. Seventy eight percent of those requested returned the form. The 3,743 women participating in the study were from ages 15 to 44 (with about equal numbers from each age group). In Denmark, all girls are required to keep track of their menstrual cycles during one of the early high school grades. Many of the women in this study had continued to keep such a record.

The Danish answer to the normal menstrual cycle length is 23 to 35 days long (this is from the shortest five percent to the longest 95 percent of cycles). However, cycles got shorter with increasing age so that the normal length was from 23 to 30 days by age 44 (2).

The answer to the question of variability was surprising. A cycle length variation of more than two weeks occurred for 29 percent of women. This variability was more common for the poorest women in the population.

Recently an international conference of experts was held to discuss menstrual cycles and to get rid of words like "polymenorrhea" for cycles that were too short (3). These experts decided menstrual cycles shorter than 24 days apart were "frequent," cycles 24-38 days apart were "normal," and cycles longer than 38 days were "infrequent." They also decided that "regular" meant a variation of plus or minus two to 20 days; "irregular" menstrual cycles varied by more than 20 days.

Well, that's a long-winded, complicated answer to your question. I think the best thing for each menstruating woman to do is to keep track for herself. Then she'll know what's normal for herself even if the experts don't agree.

Hope this is helpful,

All the best,

Jerilynn

 

Reference List

  1. Abraham GE. The normal menstrual cycle. In Givens JR, ed. Endocrine causes of menstrual disorders, pp 15-44. Chicago: Year Book Medical Publishers, Inc, 1978.
  2. Munster K, Schmidt L, Helm P. Length and variation in the menstrual cycle--a cross-sectional study from a Danish county. Br.J.Obstet.Gynaecol. 1992;99:422-9.
  3. Fraser IS, Critchley HOD, Munro MG, Broder M. Can we achieve international agreement on terminologies and definitions used to describe abnormalities of menstrual bleeding? Human Reproduction 2007;22:635-43.

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