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American Journal of Epidemiology Advance Access originally published online on August 23, 2006
American Journal of Epidemiology 2006 164(10):1003-1011; doi:10.1093/aje/kwj282
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American Journal of Epidemiology Copyright © 2006 by the Johns Hopkins Bloomberg School of Public Health All rights reserved; printed in U.S.A.

Original Contribution

From Menarche to Menopause: Trends among US Women Born from 1912 to 1969

Hazel B. Nichols1, Amy Trentham-Dietz1,2, John M. Hampton1, Linda Titus-Ernstoff3, Kathleen M. Egan4, Walter C. Willett5 and Polly A. Newcomb6

1 University of Wisconsin Comprehensive Cancer Center, Madison, WI
2 Department of Population Health Sciences, University of Wisconsin, Madison, WI
3 Dartmouth Medical School, Lebanon, NH
4 Vanderbilt University, Nashville, TN
5 Harvard School of Public Health, Boston, MA
6 Fred Hutchinson Cancer Research Center, Seattle, WA

Correspondence to Hazel Nichols, University of Wisconsin Comprehensive Cancer Center, Room 307, WARF Building, 610 Walnut Street, Madison, WI 53726 (e-mail: hbnichols{at}wisc.edu).

Received for publication August 18, 2005. Accepted for publication April 12, 2006.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
The authors investigated secular trends in age at menarche, age at menopause, and reproductive life span within a population-based cohort of US women. Study subjects were 22,774 women selected randomly as controls for a case-control study. Eligible controls were residents of Wisconsin, Massachusetts, or New Hampshire born between 1910 and 1969. Subjects completed telephone interviews in 1988–2001 and answered questions regarding reproductive and lifestyle factors. Birth cohorts were created using 5- and 10-year periods, and statistical comparisons were performed with analysis of variance. The mean age at menarche decreased by approximately 6 months for those born between 1910 and 1949 (13.1 vs. 12.7 years; p < 0.001), with a subsequent increase to 13.0 years among women born between 1960 and 1969 (p < 0.001). Among naturally menopausal women aged 60 or more years who reported never use of postmenopausal hormone therapy, the authors observed a 17-month increase in the mean age at menopause for those born between 1915 and 1939 (49.1 vs. 50.5 years; p = 0.001) after adjustment for potential confounders. They also observed an increase in the average number of reproductive years (subtracting age at menarche from age at natural menopause), from 36.1 years among women born between 1915 and 1919 to 37.7 years among the 1935–1939 cohort (p = 0.0001). These findings have implications for women's lifetime exposure to circulating endogenous hormones.

case-control studies; hormones; menarche; menopause; reproduction; women


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Age at menarche and age at menopause mark important points of physical and cultural maturation. These ages can be predictive of health and disease status; for example, breast cancer risk decreases by 5 percent with each 1-year increase in the age at menarche (1Go). Delays in age at menarche may alternatively have detrimental effects on other health outcomes, such as bone fracture and mineral density (2Go–5Go). In addition, age at menopause has been associated with risk of heart disease and osteoporosis, as well as with all-cause mortality (5Go–11Go).

In the United States, a downward trend in the average age at menarche has been generally accepted (12Go). Investigations of a possible secular trend in age at natural menopause have produced varied results; studies have proclaimed that there is no evidence of a secular trend (13Go–15Go) or that a secular trend may exist (16Go), while still others argue that available data are not sufficiently reliable to identify the existence of such a trend (17Go, 18Go). Some challenges to these evaluations include, but are not limited to, variation in the definition of menopause, differences in natural and surgical menopause, reliability and accuracy of self-reported information (often concerning events that happened decades ago), and the choice of appropriate statistical techniques.

By addressing both age at menarche and age at menopause, our study was able to define a measure of reproductive life span. Changes in both the average ages at menarche and menopause have implications for the total length of exposure to high levels of circulating estrogens during reproductive years (19Go). Our study uses interview data from 22,774 women in Massachusetts, New Hampshire, and Wisconsin to evaluate changes over time in the average age at menarche, age at menopause, and total reproductive life span.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
Study population
This pooled analysis was based on the control women who participated in three consecutive population-based case-control studies conducted as part of the Collaborative Breast Cancer Study, a project undertaken by investigators at the University of Wisconsin-Madison, the Harvard School of Public Health, and Dartmouth Medical School. Each study was conducted according to institutionally approved protocols. Participant recruitment occurred during 1988–1991, 1992–1995, and 1997–2001. The present analysis is restricted to residents of New Hampshire, Massachusetts (excluding metropolitan Boston), or Wisconsin born between 1910 and 1969 (n = 22,774). Further information regarding these studies has been published elsewhere (20Go).

Subject identification
Community controls were enrolled by a stratified random selection from population lists of licensed drivers and Medicare beneficiaries (women aged 65 or more years). Controls were selected to match, in 5-year intervals, the age distribution of eligible and enrolled breast cancer cases; approximately one control was recruited per case. Inclusion criteria required that all controls have a listed telephone number. Of 28,621 women identified as potential controls, 376 (1.4 percent) were deceased, 4,730 (17.0 percent) refused, 664 (2.4 percent) could not be located, and 37 provided information deemed unreliable by the interviewer. Overall, 79.8 percent (n = 22,851) of population controls participated in the study.

Data collection
All women completed a structured 25- to 45-minute telephone interview of reproductive characteristics and lifestyle factors. Questions regarding age at menarche were identical during all enrollment periods.

In the first of the three pooled studies (study I: 1988–1991), natural menopause was defined by self-report as the absence of menses for 12 consecutive months not due to radiation, hysterectomy, or drug use. In the most recent two of the three studies (study II: 1992–1995; study III: 1997–2001), natural menopause was defined as the absence of menses for 6 consecutive months not due to surgery, chemotherapy, radiation, or other reasons.

Statistical analysis
Birth cohort characteristics were computed using least-squares means (SAS PROC GLM) with SAS, version 9.1, software (SAS Institute, Cary, North Carolina). Tukey's test for comparing mean values of age at menarche, menopause, and number of reproductive years was used to account for multiple comparisons (21Go). We evaluated several covariates as potential confounders. These variables included state (Wisconsin, Massachusetts, New Hampshire), smoking status (never, former, current), marital status (single, married, divorced or separated, widowed), parity (continuous), age at last birth (continuous), body mass index (continuous), height (continuous), education (less than high school, high school diploma, some college, 4-year college degree or higher), oral contraceptive use (ever/never), postmenopausal hormone use (never, former, current), and alcohol consumption (continuous). Current cigarette smoking (yes/no) has been associated with age at menopause and was considered as a potential covariate in the present analyses (13Go, 14Go, 22Go). State of residence was also included in all models but did not obtain statistical significance. As the study population was 96 percent White (n = 21,958), we did not evaluate age at menarche or menopause associations by race. Those variables reaching statistical significance (p < 0.05) in preliminary models were included in multivariate models; all p values are two sided. Birth cohorts were constructed using 5- and 10-year intervals.

Age at menarche
We excluded women with age at menarche less than the first percentile (9 years) or greater than the 99th percentile (17 years) from analysis (n = 39 and n = 158, respectively). Only study participants aged 20 or more years who provided a single-year age at menarche within the above range were included in the final models (n = 22,066). In looking at the average age at menarche, we utilized 10-year birth cohorts to account for the wide range of birth years available for analysis (1912–1969). Models for age at menarche included state of residence as a covariate. At the time of interview, participants' ages ranged from 20 to 80 years of age.

Age at natural menopause
We restricted age at menopause models to naturally menopausal women aged 60 or more years. Women whose self-reported age at menopause was less than the first percentile (28 years) or greater than the 99th percentile (59 years) were excluded from analyses (n = 117 and n = 38, respectively). Women born in 1910–1939 were categorized into 5-year birth cohorts. In preliminary models (adjusted for state of residence and current cigarette smoking only), marital status, oral contraceptive use, parity, age at last birth, body mass index, height, education, and postmenopausal hormone use were individually associated with age at menopause (p < 0.05). Age at menarche and alcohol consumption were not associated with age at menopause. Oral contraceptive use was not included in multivariable models because of the extremely low prevalence of use in early birth cohorts. All final models were adjusted for state, current cigarette smoking, education, parity, age at last birth, height, and body mass index and restricted to never users of postmenopausal hormones. Adjustment for marital status did not appreciably change mean age estimates but increased the standard error, so it was not included in multivariable models. After all the exclusions, 4,767 participants (aged 60–80 years at the time of interview) were available for analysis.

Measure of reproductive life span
We calculated the maximum potential number of reproductive years by subtracting a woman's age at menarche from her age at natural menopause. Final models were restricted to naturally menopausal women aged 60 or more years who had never used postmenopausal hormones and were adjusted for enrollment state, current cigarette smoking, education, parity, age at last birth, height, and body mass index. After all the exclusions, 4,657 women aged 60–80 years at interview remained for analysis.

Reliability substudy
We reinterviewed a sequential sample of study participants in study II to evaluate the reliability of the questionnaire. Intraclass correlation coefficients and 95 percent lower confidence limits were estimated to evaluate the reproducibility of questionnaire items (23Go). After an average of 3.5 months (range: 2–6 months), 195 study subjects (74 percent) completed a second interview. Reproducibility of the interview was high; the intraclass correlation coefficient was 0.91 (lower confidence limit: 0.89) for age at menarche and 0.82 (lower confidence limit: 0.76) for age at natural menopause.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
Some characteristics of participants differed by state (table 1). Women from Wisconsin were less likely to have ever smoked and to have graduated from college than were women in the two New England states. They were also more likely to be of higher body mass.


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  		TABLE 1. Study population characteristics according to state, United States, 1988–2001

 
Pooled and state-specific mean ages at menarche for women born in 1910–1969 are presented in table 2 (n = 22,066). We observed a U-shaped distribution for age at menarche among women aged 20 or more years and born between 1910 and 1969 (figure 1). The average age at menarche declined by approximately 6 months from 1910 to 1949. The mean age at menarche among women born in 1910–1919 was 13.1 years. In subsequent decades (1920–1949), the mean was 12.9, 12.8, and 12.7 years, respectively. We compared the mean age at menarche for each decade and found significant differences between birth cohorts after adjustment for multiple comparisons. Women born in 1910–1919 and 1920–1929 had a significantly older mean age at menarche compared with each subsequent 10-year birth cohort (p < 0.001) except 1960–1969. The decline in the average age at menarche from 12.8 years among women born in 1930–1939 to 12.7 years among women born in 1940–1949 also achieved statistical significance (p = 0.008).


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  		TABLE 2. Mean age at menarche among women aged 20 or more years according to birth cohort and state, United States, 1988–2001*

 

Figure 1
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  		FIGURE 1. Mean (standard error) age at menarche by birth cohort among women aged 20 or more years living in Massachusetts, New Hampshire, or Wisconsin in 1988–2001. Means are adjusted for state of residence.

 
We observed an increase in mean age at menarche among women born in 1950–1959; this finding was also observed in the 1960–1969 cohort. Average age at menarche increased significantly from 12.7 years in the 1940 cohort to 13.0 years in the 1960–1969 cohort (p = 0.001). This increasing trend in age at menarche starting with the 1950 cohort was observed for residents of Massachusetts and Wisconsin. In New Hampshire, age at menarche declined steadily across birth cohorts (table 2).

We observed increases in the proportion of parous women (83 percent vs. 91 percent; p < 0.001), women categorized as current smokers (8 percent vs. 25 percent; p < 0.001), and women who used oral contraceptives (1 percent vs. 51 percent; p < 0.001) between the 1910–1914 and 1935–1939 birth cohorts (data not shown). Moreover, from 1910–1914 to 1935–1939, we observed a consistent increase in the proportion of women who graduated from college (8–18 percent; p < 0.001) and women categorized as obese (≥30 kg/m2) (11–18 percent; p = 0.004).

In our study, age at menopause increased according to higher education, parity, body mass index, and never-smoker status (data not shown). In models adjusted for these and other covariates, we observed a 17-month increase in the mean age at menopause from 1915 to 1939 (n = 4,767; figure 2). The average age at menopause among women born in 1910–1914 was 49.9 years. In subsequent 5-year birth cohorts (1915–1939), the mean age was 49.1, 49.5, 50.1, 50.1, and 50.5 years, respectively (table 3). After adjustment for multiple comparisons, women born in 1915–1919 had a significantly lower average age at menopause compared with women born in 1925–1929, 1930–1934, and 1935–1939 (p < 0.01). Similarly, women born in 1920–1924 had a significantly younger average age at menopause compared with women born in 1935–1939 (p < 0.05). Differences between all the other 5-year birth cohorts were not statistically significant (p ≥ 0.05).


Figure 2
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  		FIGURE 2. Mean (standard error) age at natural menopause by birth cohort among women aged 60 or more years living in Massachusetts, New Hampshire, or Wisconsin in 1988–2001. Means are adjusted for state of residence, cigarette smoking, education, parity, age at last birth, height, and body mass index. Analysis was restricted to never users of postmenopausal hormone therapy.

 

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  		TABLE 3. Mean age at menopause and average reproductive life span among naturally menopausal women aged 60 or more years according to birth cohort and state, United States, 1988–2001*

 
We calculated the number of reproductive years for naturally menopausal women aged 60 or more years who had never used postmenopausal hormones (n = 4,657; figure 3). In adjusted models, women born in 1910–1914 had an average reproductive life span (number of years between menarche and menopause) of 36.9 years. Women born in 1915–1919 had the lowest average number of reproductive years (36.1 years). The average number of reproductive years increased in subsequent birth cohorts to 36.5 years for those born in 1920–1924, 37.2 years in 1925–1929 and 1930–1934, and 37.7 years in 1935–1939 (table 3). Women born in 1915–1919 had a significantly shorter average number of reproductive years compared with women born in 1925 and beyond (p < 0.001). Members of the 1920–1924 birth cohort also had a statistically shorter mean reproductive life span compared with women born in 1925–1929 and 1935–1939 (p < 0.05).


Figure 3
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  		FIGURE 3. Mean (standard error) length of reproductive life span among women aged 60 or more years living in Massachusetts, New Hampshire, or Wisconsin in 1988–2001. Means are adjusted for state, cigarette smoking, education, parity, age at last birth, height, and body mass index. Analysis was restricted to never users of postmenopausal hormone therapy.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
Our study is one of the first to observe a recent reversal in the downward trend of age at menarche over the last century within developed countries (24Go–27Go). Roberts and Dann (25Go) examined the mean menarcheal age among students at University College, Swansea (Wales, United Kingdom), born in 1940–1951. During the first part of the decade, the mean age at menarche decreased from 12.94 to 12.46 years; it then increased to 12.79 years among women born in 1951 (25Go). This finding is in line with our own observation of an increase in age at menarche beginning with the 1950–1959 birth cohort. The authors later conducted a similar investigation at the University of Warwick (Coventry, United Kingdom) among students born in 1952–1967 and observed a continuing increase in the mean age at menarche and a significant positive association between menarcheal age and year of birth (p < 0.001) (26Go).

Interestingly, the upward trend in mean age at menarche in our study population was restricted to women living in Wisconsin and Massachusetts; the downward slope in mean age at menarche appeared to continue among women living in New Hampshire, perhaps because of the relatively small numbers of women in that state's cohort. Although the overall increase in mean age at menarche from 12.7 to 13.0 years during 1940–1969 achieved statistical significance (p = 0.001), the number of women in the 1960–1969 birth cohort was also relatively small (n = 433). We calculated mean ages at menarche for the earlier birth cohorts using models with and without subjects from the 1960–1969 cohort and found identical results.

Results among younger women may or may not adhere to this upward trend; subsequent birth cohorts (1970–present) likely have a greater prevalence of childhood overweight, which could exert a downward driving force on the average age at menarche (28Go, 29Go). Findings from the Third National Health and Nutrition Examination Survey cohort (NHANES III; 1988–1994) support a continuation in the downward trend in age at menarche. Anderson et al. (30Go) reported an average age at menarche of 12.6 years among White girls aged 10–15 years; these girls were likely born in 1973–1984.

The initial downward slope that we observed in the average age at menarche is in agreement with several previous studies (12Go, 31Go–33Go). Our models of age at menarche were restricted to women aged 20 or more years and adjusted for state of residence. An association between earlier ages at menarche and increasing height (an indicator of early life nutritional environment) has been repeatedly observed (28Go, 29Go, 34Go–36Go). Improvements in nutritional status are often cited as the predominant factor in the acceleration of age at menarche. In addition to changes in the age at menarche, we observed a small but consistent and statistically significant (p < 0.05) increase in the average adult height by 10-year birth cohort. Among women born in 1910–1919, the average adult height was 1.62 m (~5 feet, 3 inches), compared with 1.65 m (5 feet, 5 inches) among women born in 1960–1969 (p < 0.0001).

An early 20th century study of factors associated with age at menopause in Pittsburgh, Pennsylvania, reported a mean age at natural menopause of 47.1 years (37Go). A year later, a 1919 investigation of 200 women from Pennsylvania hospitals reported the mean age at natural menopause as 47.9 years, reportedly older than means of 42–44 years in the late 1800s (38Go). During the last 30 years, the mean reported age at menopause among US women has ranged from 48.8 to 51.0 years (6Go, 8Go, 10Go, 13Go, 39Go–41Go).

In numerous studies, cigarette smoking has been negatively associated with age at natural menopause (8Go, 13Go, 15Go, 22Go, 39Go, 42Go). Current smokers, on average, have a 1.5- to 2-year earlier menopause (14Go, 19Go, 22Go, 40Go). Higher levels of education (19Go, 43Go–45Go), parity (8Go, 39Go–41Go, 43Go, 46Go), and body mass index (47Go) have been associated with later ages at menopause. In our data, we observed each of the expected trends for age at menopause in relation to smoking, education, parity, and body mass index. We adjusted all final models of age at menopause for these important covariates and restricted analyses to never users of postmenopausal hormone therapy.

We observed an increase in the average age at menopause, from 49.1 years in the 1915–1919 cohort to 50.5 years in the 1935–1939 birth cohort. This finding is in agreement with some previous studies (16Go, 48Go), but it is not supported by others (14Go, 15Go, 49Go). The latter studies did not show a significant change in mean age at menopause over time; we are unaware of any study that suggests a negative trend in age at menopause. There are many speculated causes of delays in age at menopause, but no one hypothesis has gained widespread support. Potential delays in age at menopause may be associated with genetic, social, environmental, and/or hormonal exposure associations (13Go, 19Go).

In comparison with women in the 1910–1914 birth cohort, women born in 1935–1939 were more likely to be parous, to smoke cigarettes, to receive a college education, to have a greater body mass index, and to use oral contraceptives. Secular changes in these and other characteristics likely exert opposing effects on age at menopause; in our data, the net result appeared to be a modest increase in the average age at menopause. Although oral contraceptive use was not included as a covariate in our final models of age at natural menopause, models that adjusted for oral contraceptive use as either a binary (ever/never) or continuous (duration-months) variable produced similar estimates and patterns of effect.

On the basis of earlier menarche and later natural menopause, we observed a positive trend in the average number of reproductive years. The mean reproductive life span increased from 36.1 years among women born in 1915–1919 to 37.7 years among women born in 1935–1939 after adjustment for state, smoking, education, parity, age at last birth, height, and body mass index. We were unable to determine whether this trend continued despite the potential increase in age at menarche among later birth cohorts; members of the 1940–1949, 1950–1959, and 1960–1969 cohorts were too young to analyze age at menopause or to calculate reproductive life span. Increases in women's lifetime exposure to circulating endogenous hormones have potential implications for heart disease, osteoporosis, cancer risk, and all-cause mortality (10Go, 41Go).

Within a cohort of freshman students at the University of Minnesota enrolled during 1934–1937, Treloar (24Go) reported means of 13.6, 49.5, and 35.9 years for age at menarche, age at menopause, and reproductive life span, respectively. Assuming that the average college freshman is 18 years of age, this cohort was born during 1916–1919. Results are comparable with our own; in the 1915–1919 birth cohort, we observed a mean age at menarche of 13.1 years, a mean age at menopause of 49.1 years, and an average reproductive life span of 36.1 years.

Our study findings are strengthened by our ability to adjust for known confounders, to present data from three US states, and to clearly distinguish natural from surgical menopause. In our reliability substudy, participants' responses to questions regarding age at menarche and age at menopause were highly reproducible. Despite the large size of our study population and the use of standardized instruments, some limitations should be considered when interpreting these results. Approximately 80 percent of eligible controls participated in each of the parent case-control studies. These high response rates provide reassurance that the present findings should be generally applicable to eligible women in the study populations. However, our results may not be generalizable to women who have used postmenopausal hormones, women with surgical menopause, women of other racial groups, and/or women who do not survive to age 60. There is some evidence that early menopause is associated with increased all-cause mortality (11Go); however, results from a recent large prospective cohort indicated that this increase was limited to women who reached the age of 75 or more years because of the decreased risk of cancer mortality among women younger than age 75 with earlier ages at natural menopause (50Go).

Our results remained unchanged when performing our analyses both with and without records from 875 control women who were excluded from studies I–III because of a previous breast cancer diagnosis. Controls with a self-reported personal history of breast cancer were included in all final models of the present analysis.

Investigations of secular trends of age at menopause have been subject to a number of methodological difficulties, including small samples (particularly in hospital studies (17Go, 51Go)), a lack of discrimination between surgical and natural menopause (5Go, 17Go, 52Go), and variation in the definition of menopause (15Go, 52Go). Among women in our study, 25–28 percent (depending on the state of enrollment) were classified as surgically menopausal because of hysterectomy and/or bilateral oophorectomy and were thus excluded. This is similar to the 25–30 percent of US women reported to have a surgical menopause in 1960–1962 (51Go).

Historically, there is great variation in the length of amenorrhea used to define menopause. In our analysis, natural menopause was defined by 12 months of amenorrhea in the first case-control study (1988–1991) and 6 months in the two most recent studies (1992–1995; 1997–2001). A 9-month definition was used in Frommer's 1964 investigation of women admitted to the Royal Free Hospital in London (48Go); other investigators have used self-determined menopause (22, 41, 53, 54) or periods of amenorrhea for as short as 3 months (43Go) or as long as 5 years (55Go). The World Health Organization defines natural menopause as the absence of menses for 12 consecutive months without an obvious cause (such as pregnancy, lactation, or surgery) (56Go); the age at last menstrual period is calculated retrospectively (10, 13, 15, 19, 24, 39, 40, 42, 44, 45, 52, 57, 58). We compared the distribution of reported ages at menopause using the 6-month and 12-month definitions in study I and study II for each 5-year cohort from 1910 to 1939. In our data, no significant differences in the distribution of ages at menopause were observed by menopausal definition after adjusting for study, state, cigarette smoking, education, parity, age at last birth, height, and body mass index.

We analyzed age at menopause among women aged 60 or more years to avoid incurring bias in the mean age by excluding women whose menopause had not yet occurred (51Go). Although age at menarche is generally well recalled (59Go–62Go), the reliability of self-reported ages of natural menopause has been questioned (15Go, 54Go, 63Go, 64Go). Some evidence suggests that, because of errors in recall and visible digit preference (specifically the use of zero and five as terminal digits (16Go, 51Go)), the use of self-reported age at menopause may result in an underestimate in the true mean age at menopause (15Go, 16Go, 49Go).

Bean et al. (62Go) reported that, among their sample of 49 women with known age at menopause (who were asked to recall age at menopause an average of 7.6 years later), 45 percent recalled their age at menopause accurately, and 76 percent reported age at menopause within 1 year of the actual age. The mean recalled age at natural menopause was 0.6 years older than the actual mean age (62Go). An investigation of the accuracy and reproducibility of self-reported age at menopause in the Nurses' Health Study found that 82 percent (n = 4,265) of women accurately recalled age at natural menopause within 1 year when evaluated 2 years later (53Go). Variation in both self-reported age at menarche and menopause increases with increasing time since menopause (49Go, 53Go, 62Go). In our study, women born in the earliest birth cohort had a greater average length of recall compared with women in the most recent birth cohort (27.9 years vs. 11.0 years for age at menopause). This may have introduced greater variability in the mean age of menopause calculated for earlier, compared with later, birth cohorts.

The "status quo" method of data collection and analysis is an alternative to using self-reported ages at menopause (13Go, 16Go). This method uses probit analysis to calculate the median age at menopause without knowing individual ages at menopause (15Go, 17Go, 48Go). Since our data were collected over a 13-year period (1988–2001), rather than during each decade from 1950 to 2000, we were unable to use this analysis technique to address trends over time. For this reason, despite our ability to categorize a wide age range of women as pre- or postmenopausal, we have utilized self-reported age at menopause for our analyses.

In conclusion, among women born from 1910 to 1949, we observed a decreasing average age at menarche, increasing mean age at menopause, and subsequent increase in the average reproductive life span. Later birth cohorts (1950–1959 and 1960–1969) showed evidence of an increase in the average age at menarche. Because of the young ages of women in these cohorts, we were unable to determine whether the positive trends in age at menopause and reproductive life span remained. Secular changes in age at menarche and menopause may have implications for women's lifetime exposure to circulating endogenous hormones and subsequent risk of heart disease, osteoporosis, cancer, and all-cause mortality (3Go–8Go, 10Go).


   ACKNOWLEDGMENTS
 
This research was supported by grants from the National Cancer Institute (CA47147, CA47305, CA069664, and CA14520).

The authors would like to thank Drs. E. Robert Greenberg and John D. Baron for their advice and support on this project, as well as all study staff and students.

Preliminary data from this research were presented at the annual meeting of the Society for Epidemiologic Research, Salt Lake City, Utah, June 2004.

Conflict of interest: none declared.


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

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