Editing Menarche (section)

====Changes in time of average age====
[[File:Acceleration1.jpg|thumb|upright=1.8|Secular trend in decrease in age of menarche in Western European and North American girls; Boaz (1999)<ref>Boaz, N.T. (1999). ''Essentials of biological anthropology.'' Prentice Hall, New Jersey, {{ISBN|0-13-080793-1}}.</ref>]]

There were few systematic studies of timing of menarche before the second half of the 20th century. Most older estimates of average onset of menarche were based on observation of a small, homogeneous, non-representative sample of the larger population, or based on recall by adult women, which is susceptible to error. Most sources agree that the average age of menarche in girls in modern societies has declined, though the reasons and the degree remain subjects of study. 

From the sixth to the 15th centuries in Europe, most women reached menarche at about 14, between the ages of 12 and 15.<ref>{{cite journal |author1=Amundsen Darrel |author2=Dreis Carol Jean | year = 1973 | title = The Age of Menarche in Medieval Europe | journal = Human Biology | volume = 45 | issue = 3| pages = 363–68 |pmid=4584336 }}</ref> The average age of menarche dropped from 14-15 years in the early 20th century to 12-13 years in the present, but girls in the 19th century had a later age of menarche (16 to 18 years) compared to girls in earlier centuries.<ref>{{cite web |title=Average age of Menarche in History |work=British Medical Journal |url=https://www.bmj.com/content/322/7294/1095.full}}</ref> A large North American survey reported a 2–3 month decline from the mid-1970s to the mid-1990s.<ref name=AndersonEtAl2003>{{cite journal |vauthors = Anderson SE, Dallal GE, Must A |title = Relative weight and race influence average age at menarche: results from two nationally representative surveys of US girls studied 25 years apart |journal = Pediatrics | volume = 111 |issue = 4 Pt 1 | pages = 844–50 | date = April 2003 | pmid = 12671122 | doi = 10.1542/peds.111.4.844 }}</ref> A 2011 study found that each 1&nbsp;kg/m<sup>2</sup> increase in childhood [[body-mass index]] (BMI) can be expected to result in a 6.5% higher absolute risk of early menarche (before age 12 years).<ref>{{cite journal | vauthors = Mumby HS, Elks CE, Li S, Sharp SJ, Khaw KT, Luben RN, Wareham NJ, Loos RJ, Ong KK | display-authors = 6 | title = Mendelian Randomisation Study of Childhood BMI and Early Menarche | journal = Journal of Obesity | volume = 2011 | page = 180729 | year = 2011 | pmid = 21773002 | pmc = 3136158 | doi = 10.1155/2011/180729 | doi-access = free }}</ref> This is called the '''secular trend'''.<ref name="Strickland2001">{{cite book|editor-first1=Bonnie B.|editor-last1=Strickland|title=The Gale Encyclopedia of Psychology|url=https://books.google.com/books?id=kSoZAQAAIAAJ&pg=PA528 |year=2001 |chapter=Puberty |chapter-url=<!-- not a Google Books -->|publisher=Gale Group|isbn=978-0-7876-4786-5|page=528 |edition=Second |quote=The sequence and age range of the developmental changes associated with puberty can vary widely. Although most children begin puberty between the ages of 10 and 12, it can start at any age from 8 to 16. The most obvious determining factor is gender; on average, puberty arrives earlier for girls than boys. Heredity also appears to play an important role. Compared to an overall age range of nine to 18 for menarche, the age difference for sisters averages only 13 months and for identical twins, less than three months. Body weight is a factor as well: puberty often begins earlier in heavier children of both sexes and later in thinner ones. The onset of menstruation, in particular, appears to be related to amounts of body fat. Girls with little body fat, especially athletes, often start menstruating at a later than-average age. Over the past 100 years, puberty has tended to begin increasingly early in both sexes (a phenomenon called the ''secular trend'').}}</ref><ref>{{cite book |first=Laurence |last=Steinberg|editor-first1=Bonnie B.|editor-last1=Strickland  |title=The Gale Encyclopedia of Psychology |year=2000 |chapter=Adolescence |chapter-url=|publisher=Gale Group|isbn=0-7876-4786-1 |page=11 |edition=Second |quote=The timing of physical maturation varies widely. In the United States today, menarche, the first menstrual period, typically occurs around age 12, although some youngsters start puberty when they are only eight or nine, others when they are well into their teens. The duration of puberty also varies greatly: eighteen months to six years in girls and two to five years in boys.}}</ref>

In 2002, fewer than 10% of U.S. girls started to menstruate before 11 years of age, and 90% of all U.S. girls were menstruating by 13.75 years of age, with a median age of 12.43 years. This age at menarche is not much different (0.3 years earlier) than that reported for U.S. girls in 1973. Age at menarche for non-Hispanic black girls was significantly earlier than that of white girls, whereas non-white Mexican American girls were only slightly earlier than white girls.<ref name=Chumlea>{{cite journal | vauthors = Chumlea WC, Schubert CM, Roche AF, Kulin HE, Lee PA, Himes JH, Sun SS |title = Age at menarche and racial comparisons in US girls | journal = Pediatrics | volume = 111 | issue = 1 | pages = 110–13 | date = January 2003 | pmid = 12509562 | doi = 10.1542/peds.111.1.110 }}</ref>