Length
Although many people believe the average menstrual cycle takes 28 days, a large study of more than 30,000 cycles from more than 2300 women showed that the mean cycle length was 29.1 with a standard deviation of 7.5 days and a 95% prediction interval of between 15 and 45 days. In that study, the subset of data with cycle lengths between 15 and 45 days had an average length of 28.1 days with a standard deviation of 4 days. A smaller study of 140 women performed in 2006 found a mean cycle length of 28.9 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. Usually, length variation between eight and 20 days in a woman is considered as moderately irregular menstrual cycles. Variation of 21 days or more is considered very irregular.
A 1979 study of 305 women found that approximately one-third of the subjects had lunar period cycles in length, i.e., a mean cycle length of 29.5 days plus or minus 1 day. Almost two-thirds of the subjects started their cycle in the brighter half of the lunar cycle, significantly more than would be expected by random distribution. Another study found a statistically significant number of menstruations occurred around the new moon.
Read more about this topic: Menstrual Cycle
Famous quotes containing the word length:
“Punishment followed on a grand scale. For ten days, an unconscionable length of time, my father blessed the palms of his child’s outstretched, four-year-old hands with a sharp switch. Seven strokes a day on each hand; that makes one hundred forty strokes and then some. This put an end to the child’s innocence.”
—Christoph Meckel (20th century)
“It was inspiriting to hear the regular dip of the paddles, as if they were our fins or flippers, and to realize that we were at length fairly embarked. We who had felt strangely as stage-passengers and tavern-lodgers were suddenly naturalized there and presented with the freedom of the lakes and woods.”
—Henry David Thoreau (1817–1862)
“We praise a man who feels angry on the right grounds and against the right persons and also in the right manner at the right moment and for the right length of time.”
—Aristotle (384–322 B.C.)