There are a couple things in this question I want to address. First, if it is a coincidence it is not connected. In science there is a concept of "coincidence or correlation does not imply causation or connection". There is a great humor blog called Spurious Correlations that shows that you can make anything correlate.
That aside, the second and more important concept to address is that this assumes all women have the same menstrual cycle. This is an assumption that is often made but not reality. What it comes down to is that this concept is comparing two estimates as if they are real, measured and accurate numbers. I think this is what we will address, as well as the moon cycle itself.
How long does it take the moon to orbit the Earth?
This in itself is a complicated question because the moon phase appearance is influenced by the moon orbiting the Earth and the Earth orbiting the Sun. The moon orbits the Earth in just over 27 days. However, it is 29 and a half days for the moon to pass through all the phases. (Discussion of Lunar cycles) Which of these would we consider the moon cycle? The 28 days we usually hear is an average of these two time lengths. (We will see in this discussion that a lot of common knowledge on the subject is based in comparing multiple averages and not real numbers.) In truth, every common calender lists the moon cycle as 29.5 days and not the 28 that we associate with the menstrual cycle. You can read about all the calender relations and much more on lunar cycles here.The female cycle
There is a huge misnomer in common knowledge that a female cycle is fixed at a certain number of days. This is not true. The length of a female cycle changes for many reasons and may change over a woman's life time. To this discussion I think it is important to look at some of the many things that can change the length of a woman's menstrual cycle. Most women fall in a 21 to 35 day cycle while it is not uncommon for women to exceed the 35 days as well. This time period is for the average woman without the influence of medications, such as birth control, or other influences from the list we will be discussing below. Of those 21-35+ days, 2-7 days is spent bleeding. This is the numbers for adults. However, for teens and pre-teens the range is even greater and may be 21-45 days or longer between periods.Circadian Rhythm and Sleep
Our natural internal rhythms can alter the frequency and duration of a menstrual cycle. Sleeping excessively or not enough can have an impact on menstrual cycle length. Chronic insomnia can affect the length of a menstrual cycle causing a woman's normal cycle to be longer than the 35 days at the upper end of the average. Melatonin levels can change the length of a cycle too. This is important because of the use of over-the-counter melatonin for sleep which makes this influence more common than in the past. There is plenty of research that shows sleep rhythms can have such an effect on the cycle that it can impact fertility.A research paper about circadian rhythms and menstraul cycle
Eating and Weight
The amount one eats can impact length of menstrual cycle. Decrease in food due to starvation, be it external such as famine or internal such as anorexia or bulimia, can decrease the frequency of menstrual cycles. In some cases lack of food can eliminate the menstrual cycle completely. On the other side, over eating and being obese can decrease frequency of menstrual cycles. Types and quality of food can also affect menstrual cycle duration. Other things such as alcohol and caffeine consumption can alter period length.Research paper weight and sex-hormones
Research on anovulation
Bariatric surgery and menstrual/fertility issues
Exercise
Intense exercise can alter fertility and menstrual cycles as well. Called exercised induced Amenorrhea, this is a complete lack of menstrual cycle but it can also be a decrease in frequency.Medications and Illness
A large number of illnesses can affect period duration. Everything from anemia to uterine cancer can alter menstrual cycles. Medications can also affect the duration and intensity of menstrual cycles. Many mood stabilizing medications such as anti-depressants can increase or decrease the length of menstrual cycles.Diseases and conditions that are effected by or affect menstrual cycles.
Age
Young and old women both experience irregular menstrual cycle lengths. Between the ages of 25 and 40 women experience the most regularity in periods. Prior to the age of 25 menstrual cycle irregularity is related to the hormone imbalances related to sexual maturity and development. After 40 the irregularity is due to peri-menopause.Stress Levels
High stress levels can trigger less frequent or erratic menstrual cycles. At the root of this is the body mechanism, the same one in play with starvation, that signals that this is a poor time to conceive a child.This body stimulus can reduce the amount of menstrual cycles, which reduces the number of ovulations or chances to become pregnant.Stress and Menstration
Affects of stress on women's health
Research paper on job stress and menstrual length
Other considerations.
The often cited study by Winnifred B. Cutler in 1980 that is used as proof that women's cycles are related to the moon doesn't prove anything. In the study it was shown that 40% of the studied women had a period start within 2 weeks of the full moon, i.e. within half a month of the full moon. The two week time frame is very large considering this is half of the entire moon cycle. The other thing to consider is most of the women, 60%, showed no relationship at all with time of period and full moon.More reading:
Exercise and weight and menstrual cyclesMissed periods, exercise and osteoporosis
Stress, ovulation and bone health
List of everyday things that can affect periods.
Causes of Amenorrhea
The moon and activities
Moon cycle length and estrus length in multiple animals.
Do women who live together actually synchronize?
