The Age of Natural Menopause and Related Factors among the Population of the Tabari Cohort.

Background: Age of menopause is affected by several factors. In this study we aimed to identify the age of natural menopause and its related factors in a large-scaled population-based cohort study in Iran. Methods: In this study, we utilized a subset of data collected in enrollment phase of Tabari cohort study (TCS). Reproductive history and other related data were collected using structural questionnaire. Blood samples were collected form all participants. We analyzed data using chi-square, independent t-test and ANOVA as well as multivariate linear regression model. Results: Among all participants of Tabari cohort, 2753 were menopause women. The mean age of natural and induced menopause was 49.2±4.7 and 43.2 ± 6.4 respectively (P = 0.001). Our finding showed that number of pregnancy, breastfeeding duration, education, residency, thyroid disease and body mass index affect age of menopause. After adjustment for confounding variables, number of pregnancy remained significantly associated with late menopause. Conclusion: Results of the present study showed that number of pregnancy was positively associated with menopausal age.

3 to pesticides [1][2][3][4]. The World Health Organization has stated that by the year 2030, globally, there will be more than 1.2 billion women suffering from menopause along with obesity and high Body Mass Index (BMI), moreover it has been reported that every year 47 million women enter this stage of their lives [1].
Premature or early menopausal symptoms related to the progressive reductions in hormonal secretion and ovarian estrogen deficiency [5], moreover risk of cardiovascular diseases and its related morbidity are higher in women with earlier age at menopause [6].
There are varieties of definitions regarding early menopause. So if the last menstruation occurs between the age of 40 to 44 and before the mid age of 51, we could define it as premature or early ovarian function failure [7][8][9].Symptoms of menopause include hot flashes, sweating, sleep disorders, mental changes, and menstrual disorders. Later in the menopause period, vaginal atrophic changes and related sexual dysfunctions, cardiovascular diseases, osteoporosis, psychosocial changes, and muscular-articular complaints may appear. 5 These symptoms accompanied with amenorrhea, and lack of estrogen hormone [5]. Early menopause increases the incidence of cardiovascular disease and osteoporosis, in the other word for each year of delayed menopause, 2% of the mortality rate from cardiovascular disease is reduced [10].
The age of menopause significantly differ based on genetic background, ethnicity, region and country. Moreover socio-economic level, environment, lifestyle, reproductive or early childhood factors are related with menopausal age [11]. It has been reported that women in developing countries experience menopause earlier than women in developed ones [6,12,13]. In the present large-scale population based cohort study we aimed to identify the age of natural menopause and its related factors in women who lived in north of Iran.

Method 4
In the study, we utilized a subset of data which was collected for Tabari Cohort Study (TCS). TCS is part of the national cohort, Prospective Epidemiological Research Studies in IrAN (PERSIAN) [14,15]. The objective and details of TCS have been explained elsewhere [16]. Enrollment phase of the study started in June 2015 and ended in November 2017.In total 10255 individuals, aged 35-70 years, living in urban (7012) and mountainous areas of Sari (3243), Mazandaran, Iran, were enrolled in TCS.
Data collection method includes structural questionnaire and obtaining blood, urine, hair and nail samples. The questionnaire used in the study was a standardized questionnaire which details and features are described in cohort profiles [14,15]. Items of questionnaire consist of general information, socioeconomic status, occupational history, type of fuel used, characteristics of the habitat, life style, history of fertility, history of chronic diseases, drug use, familial history of diseases, oral health, physical examination, physical disabilities, sleep status, physical activity, personal habits (smoking and drinking), food frequency, food supplements, water drink, dietary habits, and exposure to pesticides. To explain the reproductive section of the questionnaire in more details, we asked women about the age at menarche, first pregnancy age, number of pregnancies and abortions, duration of breastfeeding, use of contraceptive, age of menopause (if happens) kind of menopause (natural or induced menopause).
Blood pressure and anthropometric indices were measured for all participants. All anthropometric indices include height, weight, waist and hip circumferences were measured by trained nurses and in accordance with Persian protocol [17].
Data were analyzed using SPSS, v. 24. Variables were described using percentage, minimum, maximum, mean and standard deviation. Categorical and quantitative variables compared in two groups using chi-square and independent t-test respectively. ANOVA was used to compare quantitative variables in more than two groups.

5
In order to adjust possible confounding variables we used the multivariate linear regression model. Variables with p-value less than 0.2 were considered in model, which include number of pregnancy, breastfeeding duration, educational level, marital status, residency, thyroid diseases, and BMI. It should be noted, that the correlation coefficient between the number of pregnancies and the number of live births was equal to 0.918 therefore only number of pregnancy entered the multivariate linear regression model.

Results
Among all participants of TCS, 2753 were menopause women. The mean age of menopause was 47.9±5.7 years with a minimum of 23 and a maximum of 64 years. The prevalence of normal menopause was 77.7% (2140), and the prevalence of induced menopauses (due to surgery) was 22.3% (613). The mean age of normal and induced menopause was 49.2±4.7 and 43.2 ± 6.4 respectively (P = 0.001). The average age of the menarche, number of pregnancies, number of children, the age at the first pregnancy, number of abortions, length of breastfeeding, and the age at the first marriage of participants are summarized in Table 1.
The history of infertility and contraceptive use in women who experienced menopause was 6.4% (136 of 2140) and 47.8% (1023 of 2140) respectively. We compared the mean age of menopause in different socio-economic levels (five levels with level 1 as the lowest level and level 5 as the highest socioeconomic level) using ANOVA test. The mean age of menopause at the lowest and highest socioeconomic level was 49.5 ± 5.1 and 49.1±4, respectively (P = 0.276). The mean age of menopause in women with and without history of infertility did not differ significantly (49.2±4.7versus 49.6±4.8, (P = 0.382)) .Moreover there was no significant difference between the mean age of menopause in women who used contraceptive and who did not (49.3 ± 4.7 versus 49.2 ± 4.8, P = 0.529). We compared the mean age of menopause in women with different marital status (single, 6 married, widow and divorced), our analysis did not reveal any significant difference (47.2 ± 5.5, 49.2 ± 4.6, 49.7 ± 5.4, .49.3 ± 3.2 respectively, P = 0.110). On the other hand the average age of menopause differ significantly according to the educational level (P<0.001). The mean age of menopause in women who live in mountainous region was significantly higher than women who live in urban area (49.8± 5.1 versus 48.8±4.5, P = 0.001) The correlation coefficient between menopausal age with abortion, breastfeeding duration, age at the first pregnancy, number of pregnancies, age at the first marriage, and age of menarche were calculated and e results are given in Table 1.With increasing the number of pregnancy and duration of breastfeeding, the age of menopause significantly increase (P< 0.001).
We compared the age of menopause in women with and without thyroid disease. Our results showed that the age of menopause in women with thyroid disease is significantly lower than women without thyroid diseases (48.7 ± 4.6 versus 49.3 ± 4.8, P = 0.023).
Moreover we compared the age of menopause in women with and without diabetes (49.4 ± 4.9 versus. 49.2 ± 4.7, P = 0.226) as well as women with and without hypertension (49.4 ± 5 vs. 49.2 ± 4.6, P = 0.266), our result did not show any significant differences between these groups.
We compared the age of menopause based on BMI (women divided into three groups, less than 25, 25-29.9 and greater than 30). Our finding showed that with increasing BMI, the age of menopause decrease (table 2).
After adjustment of possible confounding variables (number of pregnancy, breastfeeding duration, educational level, marital status, residency, thyroid diseases and BMI), number of pregnancy was still related with menopausal age. Multivariable regression analysis showed that with each unit increase in number of pregnancy and breastfeeding duration, 7 the age of menopause increases by 0.08 and 0.06 respectively(table 3).

Discussion
Results of our study showed that the mean age of menopause in total population was 47.9 ± 5.7 and the mean age in natural and induced menopause was 49.2±4.7 and 43.2 ± 6.4 respectively. Although the overall mean of menopause in our study is relatively similar to other province in Iran [12,13,[18][19][20], but the mean age of natural menopause in Tabari cohort population is greater than other provinces except Hamedan (49.6 ± 4.02) [21].
Comparing to other countries, age of menopause in our study is less than western world [6,22], but it is higher than Punjabi Women (47.91± 3.16) [23]. It has been reported that different factors such as environmental and genetic factors influence the age of menopause [24]. Result of our study showed educational level and residency affect the menopausal age although after adjusting for confounding variables neither of them remained significant. Natural menopause age indicates women health status as early menopause is associated with increased risk of cardiovascular disease and osteoporosis and late menopause is related with increased risk of breast and endometrial cancer [22].
Our finding also showed that among reproductive variables, number of pregnancy was related to menopausal age. This relationship remained significant after adjustment for confounding variables. Moreover education, residency, thyroid disease, and BMI affect age of menopause. Results of Kaczmarek' study in Poland showed that age at menarche, parity and use of oral contraceptives was related to age at menopause [22]. In this study we did not find any association between age at menarch and menopausal age. This association has not been reported consistently in previous studies.
Parity is often found to be positively associated with later age of menopause [22,25]. In this study there was a significant association between the number of pregnancy with age at menopause. 8 Association between marital status and menopausal age was inconsistent in previous studies [20,21].In our study the number of single participants was very low therefor we could not compare them with married participants.
Unlike the previous studies, results of present study did not show any significant relationship between socioeconomic level and menopausal age.
BMI is one of the factors related with menopausal age. In the present study women with lower BMI had significantly higher menopausal age but after adjustment of confounding variables there was no association between BMI and menopausal age. Association between BMI and age at menopause have been reported inconsistently. Some studies found no association between BMI and age of menopause [23], whereas others found higher BMI associated with later menopause [26][27][28].
We also compare age of menopause in women with and without some diseases. Among them history of thyroid disease was significantly related with menopausal age but diabetes, hypertension were not related with age at menopause. Multivariable regression analysis did not confirm association between thyroid disorder and age at menopause.
Cigarette smoking is the most established and consistently observed risk factor for younger age at menopause, with estimates of impact about one year with a clear doseresponse association [29].In the present study the number of smoker was very low, therefor no association between smoking and menopausal age was observed.
Some limitation of the present study should be considered, first reproductive history of women was self-reported of events years back in their life which can be subject to recall bias. Second one we did not include women older than 70 years old who may face more severe health related problems.

Conclusion
Results of the present study showed that number of pregnancy was positively associated 9 with menopausal age.

Availability of data and materials
The datasets used during the current study are available from the corresponding author on reasonable request.