The onset of puberty in women is marked by numerous changes, both physiological and behavioral, the most marked of which is the commencement of menstruation (1). The mean age at menarche reflects numerous health aspects of a population, including the timing of sexual maturation, growth and nutritional status, and environmental conditions (2). In the United States, the mean age at menarche decreased from more than 14 years prior to 1900 (3) to less than 12.5 years currently (2), although racial differences in maturational timing have been observed consistently in nationally representative and clinical samples, across time and using different study designs (4,5). Recently published data on the incidence of breast cancer among a large cohort of French women provide further evidence that the overall risk of breast cancer increases with younger age at menarche. Among premenopausal women, those with a history of menarche at age 11 or younger were at increased risk for breast cancer when compared with those with menarcheal onset at age 15 or older (relative risk 1.52; 95% confidence interval [CI], 1.03–2.22) (6). Secular trends in the average age at menarche may, in fact, affect the future epidemiology of breast cancer.

Puberty is often accompanied by the adoption of new behaviors such as alcohol drinking, cigarette smoking, and the use of oral contraceptives (OC) for pregnancy prevention with the commencement of sexual activity. In addition to the well-established data on the risks to health from cigarette smoking, substantial data are available on the excessive risk for cardiovascular diseases among smokers who use OC (7). A recently published report on mortality among women who used OC reveals more than a doubled risk for death from all causes among heavy smokers (8). Thus, data on trends in onset of puberty and behavioral aspects of adolescence are important to public health policy makers. However, few data on age of menarche in Israel are currently available. No population-based survey has been undertaken on this issue; a single published cross-sectional study was conducted more than 20 years ago (9).

The objectives of this study were to describe secular trends in age at menarche, at first cigarette, and at initial use of OC among a large population-based sample of young Israeli women and to assess whether changes in these parameters over time are associated with sociodemographic factors.

Data were available for 11,392 women, with a mean (± SD) age of 18.5 (± 0.37) years and a median of 18.47 years at recruitment. A trend of increasing education over time was recorded for subjects' fathers. Approximately 43.5% of the study participants were of western origin, with little variation over time (Table 1).

Reported mean age (± SD) at menarche showed a monotonic trend of decrease over time, from 13.41 (± 1.30) years for women born before 1970 (the earliest birth cohorts in the study population) to 13.03 (± 1.28) years for those born after 1978 (the latest birth cohorts in the study population) (P < .001) (Table 2). The proportion of subjects reporting menarche at age 11 or earlier was 3.3% for women born before 1970, 4.0% for women born from 1970 to 1972, 4.9% for women born from 1973 to 1975, 5.2% for women born from 1976 to 1978, and 6.3% for women born after 1978. Women born after 1978 were twice as likely as those born before 1970 to have had an early menarche (odds ratio 2.0; 95% CI, 1.41–2.82).

The study data showed a significant increase over time in the proportion of recruits reporting having ever smoked (from 23.55% in the earliest birth cohort to 32.86% in the latest [P < .001]) and in those reporting having ever used OC (from 25.49% in the earliest birth cohort to 31.24% in the latest [P < .001]) (Figure). A significant downward trend was observed in mean age at first use of OC with sequential birth cohorts, from 17.49 (± 0.81) years in the earliest cohort to 17.14 (± 1.05) years in the latest cohort (P < .001). A similar trend was present for age at first cigarette, with the mean age decreasing from 16.68 (± 1.30) years to 15.85 (± 1.36) years (P < .001).

Regression modeling showed a negative association between the recruits' year of birth and their age at menarche, indicating a downward temporal trend of 4.2 months per decade (95% CI, 3.6–5.0) in age at menarche. Comparison of regression coefficients between birth year and reported age at menarche shows similar results for women of western and eastern origins and for all categories of paternal education (Table 3). A tendency toward a stronger downward temporal trend was recorded among past smokers; however, this trend did not reach statistical significance. Year of birth was the sole factor that maintained a significant regression coefficient with age at menarche in a multivariate linear regression model that also included subject's origin, father's level of education, and smoking status.

This population-based study of young Israeli women demonstrates a trend over time of decreasing age at menarche, as indicated by younger age at menarche among girls born between 1965 and 1981. These same birth cohorts were characterized by decreasing age at first cigarette and at first use of OC and by an increasing proportion of adolescent smoking and OC use.

Age at first menstruation has shown a decreasing secular trend in many populations (12-14). A recently published study of a nationally representative sample of U.S. girls shows that for 80% of girls in the United States, menarche occurs between 11.00 and 13.75 years of age, with a median of 12.43 years (2). Although our study data show a relatively older average age at menarche in comparison with the United States, a steeper trend of reduction over time was recorded. Although data from the United States suggest an overall drop of approximately 2.5 months during the period between the late 1960s and the early 1990s (15), our data indicate a decrease of 4.2 months per decade. Scandinavian studies have shown a trend toward younger age at menarche during the second half of the 20^th century (16). A previous study undertaken in Israel in 1977 among a sample of 285 Jewish girls in grades three through eight (birth years 1969 and earlier) found a mean age of 13.29 (± 0.45) years at menarche (9). This mean was nearly identical to that found in the current study among girls of comparable birth cohorts.

The ecologic association between age at menarche and age of first cigarette may have several possible explanations. Girls who are relatively physically mature for their age may associate with older adolescents and imitate their behavior (17). Menarche at younger age may present a serious psychological stressor, resulting in early smoking in addition to other unhealthy behaviors. Several studies have suggested that hormonal changes accompanying menarche may directly contribute to changes in behavior (18,19).

This population-based study has several strengths, such as systematic sampling, large sample size, and standardized data collection methods throughout the entire study period. Because young women with severe disease or disability are exempt from military service, study data are not affected by potential secular changes in the epidemiology of such diseases in the general population. Two limitations must be noted, however. Since ultra-Orthodox Jewish and Arab adolescents are exempt from military service, no conclusions pertaining to these minority groups can be drawn from the present study. Additionally, data collection for this study was based on recall of age at menarche at recruitment rather than recorded prospectively at time of onset. However, because menarche is an easily recognizable event that is well remembered by most women, any reporting errors are expected to be both small and nondifferential because the study participants were all of the same age at recruitment and data were obtained only a few years after menstruation commenced.

It is generally thought that improvement in socioeconomic conditions and overall health are the main contributing factors to the trend toward earlier sexual maturation (20). Nonetheless, the mechanisms through which these changes occur are still unknown. Several hypotheses have been discussed in the literature, such as the "critical weight hypothesis" (21) and the leptin hypothesis (22). Other investigators have studied the role of estrogen-like substances in the environment on the timing of puberty (23). However, stabilization in the age of puberty onset over the last two decades, which has been observed in several countries (24,25), and studies finding that girls belonging to lower social strata showed a temporal trend toward reduction in the age at menarche (9,26,27) raise questions about the causality of these factors.

A recent cohort study of 17,032 women aged 25 to 39 who had used OC showed that death from all causes was more than twice as high in smokers (⩾15 cigarettes per day) than in nonsmokers and that harmful effects were already apparent in women aged 35 to 44 (26). The age at which smoking begins has been shown to influence the total number of years of smoking (28), the number of cigarettes smoked per day in adulthood, and the likelihood of quitting (29), all of which affect the risk for developing smoking-attributable disease and disability (30).

The ongoing significant growth in the proportion of adolescent girls who smoke at an increasingly younger age, combined with the rise in OC use and increasingly younger ages at menarche and initial OC use, suggest a rise not only in the prevalence of exposure to these risk factors but also in the duration of exposure. This trend implies a serious public health problem, which may translate into increased future morbidity and mortality. Given the epidemiology of smoking initiation, more public health policy and programmatic attention should be aimed at early adolescence in order to prevent or delay adolescent cigarette smoking. This approach strongly supports the Centers for Disease Control and Prevention's "Guidelines for School Health Programs to Prevent Tobacco Use and Addiction," which call for tobacco-use prevention education in kindergarten through 12^th grades (30). Health policy authorities should further investigate changes in health behavior among adolescents over time to evaluate the effectiveness of such intervention in the future.