Secular Trends And Predictors Of Age At Menarche In Chinese Adolescents

BackgroundAge at menarche is important to women.Because of its high reliability of recall,easy of observation and the high relevancies with reproductive health and social psychological problems,the ages at menarche were always used as an important monitoring index for economics and social development of countries and regions and girls health status.Previous studies have found that the average age at menarche has decreased from about 16 to 13 years over the past 180 years,with some variation across different countries.Although this trend towards earlier menarche may be slowing in industrialized countries,it is still more significant in other counties.The trend of young women in China has also shown a continuing declining,but there were few national representative researches and cohort studies.Additionally,China changed dramatically during the past few decades in many aspects,especially the socio-economic status,an important influencing factor of the onset of puberty.Thus the studies on menarche were needed in our country.Many previous studies have explored factors such as genetics,social demography,social economics,diet and lifestyle,which influence the age of menarche.However,the results are still controversial,and the mechanisms of the influencing factors have not yet been elucidated.Objectives1.To estimate secular trend in the age at menarche for Chinese girls born from 1973 to 2004.And to compare the differences between urban and rural girls and Han and minorities.2.To explore the influencing factors,including demographic,social-economic,nutritional and lifestyle characteristics,associated with age at menarche in a large sample of Chinese girls born over the past nearly three decades.And to cormpare the effects of the factors.MethodsData were collected from the China Health and Nutrition Survey(CHNS),a prospective open cohort and an ongoing nationwide health and nutrition survey,consisting of 3199 apparently healthy Chinese girls aged 6 to 18 years at entry from 1991 to 2011.Questions were filled out by the well-trained investigators with face-to-face interview and measure.Average age at menarche and its 95%confidence interval were estimated by weighted means of Kaplan-Meier survival analysis,which was used to predict the secular trend and differences between urban and rural,Han and minorities.Cox proportional hazard models were applied to identify the independent predictive factors of age at menarche.Statistically significant determinants of age at menarches in Chinese girls identified in the stepwise Cox proportional hazard models were then entered into general dominance analysis to access the relative contribution of the determinants.Main results1.The age at menarche declined from 14.25±0.17 in Chinese girls born before 1976 to 12.60010.59 in girls born after 2000,with an estimated decline of 0.51 years/decade(P<0.001).2.The mean age at menarche for rural girls was 5.76 months higher than for urban girls(13.68±0.05 vs.13.20±0.07).And the downward trend of age at menarche for rural girls was greater than for urban girls(0.62 years/decade vs.0.35 years/decade;P<0.001).3.The means and trends of age at menarche for Han girls and minorities were the same.4.Girls living in urban area,living in central area,having high body mass index(BMI),and having high-energy diet had high risks of early menarche,whereas girls living in east coast,and having high-carbohydrate diet had low risks of early menarche.5.Dominance analysis showed BMI is the most important determinant of age at menarche,followed by carbohydrate intake,year of birth,rural/urban residency and region.ConclusionsThis study found a continuous downward secular trend of age at menarche for Chinese girls born from 1973 to 2004.And the downward trends were found in both urban and rural area and in Han girls and minorities.Additional,the means and decline rates of age at menarche for rural girls were both greater than for urban girls.The results suggested that health education should be carried out in a proper period in adolescence.It also provided a basis for formulating relevant standards for adolescences.BMI is the most important determinant of age at menarche,followed by carbohydrate intake,year of birth,rural/urban residency and region.Among all variables,year of birth,urban residency and BMI are negatively associated with age at menarche,while carbohydrate intake and eastern residency are positively associated with age at menarche.In the future,we should pay more attention to the nutrition condition and diet of in the development adolescence and control the early age at menarche from the above factors.