Instruct: Given a question, retrieve relevant passages that answer the question.
Query: What is the average menstruation cycle length?
The first period usually begins between twelve and fifteen years of age, a point in time known as menarche.[5] They may occasionally start as early as eight, and this onset may still be normal.[6] The average age of the first period is generally later in the developing world and earlier in developed world. The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women and 21 to 35 days in adults (an average of 28 days[6][7][8]). Menstruation stops occurring after menopause which usually occurs between 45 and 55 years of age.[9] Bleeding usually lasts around 2 to 7 days.[6]
Most menstrual cycles have several days at the beginning that are infertile (pre-ovulatory infertility), a period of fertility, and then several days just before the next menstruation that are infertile (post-ovulatory infertility). The first day of red bleeding is considered day one of the menstrual cycle. To use these methods, a woman is required to know the length of her menstrual cycles.
Although it was evident that the pill could be used to suppress menstruation for arbitrary lengths of time, the original regimen was designed to produce withdrawal bleeding every four weeks to mimic the menstrual cycle.
Eumenorrhea denotes normal, regular menstruation that lasts for a few days (usually 3 to 5 days, but anywhere from 2 to 7 days is considered normal).[89][96] The average blood loss during menstruation is 35 milliliters with 10–80 ml considered normal.[97] Women who experience Menorrhagia are more susceptible to iron deficiency than the average person.[98] An enzyme called plasmin inhibits clotting in the menstrual fluid.[99]
Anovulation is usually associated with specific symptoms. However, it is important to note that they are not necessarily all displayed simultaneously. Amenorrhea (absence of menstruation) occurs in about 20% of women with ovulatory dysfunction. Infrequent and light menstruation occurs in about 40% of women with ovulatory dysfunction. Another potential symptom is irregular menstruation, where five or more menstrual cycles a year are five or more days shorter or longer than the length of the average cycle. Absence of mastodynia (breast pain or tenderness) occurs in about 20% of women with ovulatory problems. Also possible is increased body mass and facial hair, which is relatively easy to treat, and is often associated with PCOS, or polycystic ovary syndrome.
The menstrual cycle occurs due to the rise and fall of hormones.[2] This cycle results in the thickening of the lining of the uterus, and the growth of an egg, (which is required for pregnancy).[2] The egg is released from an ovary around day fourteen in the cycle; the thickened lining of the uterus provides nutrients to an embryo after implantation.[2] If pregnancy does not occur, the lining is released in what is known as menstruation.[2]
Weaker results relating the degree of a graph to unavoidable sets of cycle lengths are known: there is a set "S" of lengths, with |"S"| = O("n"), such that every graph with average degree ten or more contains a cycle with its length in "S" , and every graph whose average degree is exponential in the iterated logarithm of "n" necessarily contains a cycle whose length is a power of two . The conjecture is also known to be true for planar claw-free graphs and for graphs that avoid large induced stars and satisfy additional constraints on their degrees .
All female placental mammals have a uterine lining that builds up when the animal is fertile, but is dismantled (menstruated) when the animal is infertile. Some anthropologists have questioned the energy cost of rebuilding the endometrium every fertility cycle. However, anthropologist Beverly Strassmann has proposed that the energy savings of not having to continuously maintain the uterine lining more than offsets energy cost of having to rebuild the lining in the next fertility cycle, even in species such as humans where much of the lining is lost through bleeding (overt menstruation) rather than reabsorbed (covert menstruation).[2][20] However, even in humans, much of it is re-absorbed.
Many have questioned the evolution of overt menstruation in humans and related species, speculating on what advantage there could be to losing blood associated with dismantling the endometrium, rather than absorbing it, as most mammals do. Humans do, in fact, reabsorb about two-thirds of the endometrium each cycle. Some work asserts that overt menstruation occurs not because it is beneficial in itself. Rather, the fetal development of these species requires a more developed endometrium, one which is too thick to reabsorb completely. There is a correlation between species that have overt menstruation to those that have a large uterus relative to the adult female body size.[56]
"Irregular menstruation" is where there is variation in menstrual cycle length of more than approximately eight days for a woman. The term "metrorrhagia" is often used for irregular menstruation that occurs between the expected menstrual periods.
Irregular cycles or irregular periods is an abnormal variation in length of menstrual cycles. A female usually experiences cycle length variations of up to eight days between the shortest and longest cycle lengths. Lengths ranging between eight and 20 days are considered moderately irregular. Variation of 21 days or more is considered very irregular. 
In Buddhism (Theravada or Hinayana) menstruation is viewed as ""a natural physical excretion that women have to go through on a monthly basis, nothing more or less"". However, in certain branches of Japanese Buddhism, menstruating women are banned from attending temples.
In Nichiren Buddhism (Japan) menstruation is not considered a spiritual obstacle to religious practice, although a menstruating woman may choose not to bow, for comfort.
In Hinduism, menstruating women are traditionally advised rules to follow. During menstruation, women are advised not to “enter temple,work in kitchen,wear flowers, have sex, touch other males or females.” Menstruation is seen as unclean, and are often separated from place of worship or any object pertaining to it, for the length of their period.
Following are diagrams for estimating gestational age from obstetric ultrasound, by various target parameters:
The fertilization age (also called "embryonic age" and later "fetal age") is the time from the fertilization. It usually occurs within a day of ovulation, which, in turn, occurs on average 14.6 days after the beginning of the preceding menstruation (LMP). There is also considerable variability in this interval, with a 95% prediction interval of the ovulation of 9 to 20 days after menstruation even for an average woman who has a mean LMP-to-ovulation time of 14.6. In a reference group representing all women, the 95% prediction interval of the LMP-to-ovulation is 8.2 to 20.5 days. The actual variability between gestational age as estimated from the beginning of the last menstrual period (without the use of any additional method mentioned in previous section) is substantially larger because of uncertainty which menstrual cycle gave rise to the pregnancy. For example, the menstruation may be scarce enough to give the false appearance that an earlier menstruation gave rise to the pregnancy, potentially giving an estimated gestational age that is approximately one month too large. Also, vaginal bleeding occurs during 15-25% of first trimester pregnancies, and may be mistaken as menstruation, potentially giving an estimated gestational age that is too low.
Non-human primates are optimal models for human biomedical research because humans and non-human primates share physiological characteristics as a result of evolution. While menstruation is heavily associated with human females, and they have the most pronounced menstruation, it is also typical of ape relatives and monkeys. Female macaques menstruate, with a cycle length over the course of a lifetime that is comparable to that of female humans. Estrogens and progestogens in the menstrual cycles and during premenarche and postmenopause are also similar in female humans and macaques; however, only in macaques does keratinization of the epithelium occur during the follicular phase. The vaginal pH of macaques also differs, with near-neutral to slightly alkaline median values and is widely variable, which may be due to its lack of lactobacilli in the vaginal flora. This is one reason why, although macaques are used for studying HIV transmission and testing microbicides, animal models are not often used in the study of sexually transmitted infections, such as trichomoniasis. Another is that such conditions' causes are inextricably bound to humans' genetic makeup, making results from other species difficult to apply to humans.
Until 2009 it was thought that 28 cycles had spanned the 309 years between 1699 and 2008, giving an average length of 11.04 years, but research then showed that the longest of these (1784–1799) seems actually to have been two cycles, so that the average length is only around 10.7 years. Since observations began cycles as short as 9 years and as long as 14 years have been observed, and in the double cycle of 1784–1799 one of the two component cycles had to be less than 8 years in length. Significant amplitude variations also occur.
Average length (days) of estrus and estrous cycles:[23]
While the normal menstrual cycle in the human typically lasts 4 weeks (28 days, range 24–35 days) and consists of a follicular phase, ovulation, and a luteal phase, followed by either menstruation or pregnancy, the anovulatory cycle has cycle
lengths of varying degrees. In many circumstances, menstrual intervals are prolonged exceeding 35 days leading to oligomenorrhea (cycle >35 – 180 days interval), or even longer, amenorrhea. In other cases, menstruation may be fairly regular (eumenorrhea), or more frequent (intervals < 21 days), or there may be a loss of menstrual pattern (menorrhagia, dysfunctional uterine bleeding).
Irregular menstruation is a menstrual disorder whose manifestations include irregular cycle lengths as well as metrorrhagia (vaginal bleeding between expected periods).
Use of certain calendar rules to determine the length of the pre-ovulatory infertile phase allows unprotected intercourse during the first few days of the menstrual cycle while maintaining a very low risk of pregnancy. With mucus-only methods, there is a possibility of incorrectly identifying mid-cycle or anovulatory bleeding as menstruation. Keeping a BBT chart enables accurate identification of menstruation, when pre-ovulatory calendar rules may be reliably applied. In temperature-only systems, a calendar rule may be relied on alone to determine pre-ovulatory infertility. In symptothermal systems, the calendar rule is cross-checked by mucus records: observation of fertile cervical mucus overrides any calendar-determined infertility.
May 28 has symbolic meaning. May is the 5th month of the year and women menstruate an average of 5 days every month. Also, the menstrual cycle averages 28 days.
Strassmann argued menstrual synchrony defined as menstruation overlap should be quite common. For example, the expected difference by chance between two women with 28-day cycles—which is approximately the average length of menstrual cycles of women at the age —is 7 days. Considering that the mean duration of menses is 5 days and the range is 2 to 7 days, the probability of menstruation overlap by chance should be high.
Females of other species of placental mammal undergo estrous cycles, in which the endometrium is completely reabsorbed by the animal (covert menstruation) at the end of its reproductive cycle. Many zoologists regard this as different from a "true" menstrual cycle. Female domestic animals used for breeding —for example dogs, pigs, cattle, or horses— are monitored for physical signs of an estrous cycle period, which indicates that the animal is ready for insemination.
Menstruation is the most visible phase of the menstrual cycle and its beginning is used as the marker between cycles. The first day of menstrual bleeding is the date used for the last menstrual period (LMP). The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, and 21 to 31 days in adults (an average of 28 days).[2][3]
The luteal phase is the final phase of the ovarian cycle and it corresponds to the secretory phase of the uterine cycle. During the luteal phase, the pituitary hormones FSH and LH cause the remaining parts of the dominant follicle to transform into the corpus luteum, which produces progesterone. The increased progesterone in the adrenals starts to induce the production of estrogen. The hormones produced by the corpus luteum also suppress production of the FSH and LH that the corpus luteum needs to maintain itself. Consequently, the level of FSH and LH fall quickly over time, and the corpus luteum subsequently atrophies.[81] Falling levels of progesterone trigger menstruation and the beginning of the next cycle. From the time of ovulation until progesterone withdrawal has caused menstruation to begin, the process typically takes about two weeks, with 14 days considered normal. For an individual woman, the follicular phase often varies in length from cycle to cycle; by contrast, the length of her luteal phase will be fairly consistent from cycle to cycle.[91]
During pregnancy and for some time after childbirth, menstruation does not occur. The average length of postpartum amenorrhoea is longer when breastfeeding; this is termed lactational amenorrhoea.