Secular trends in menarche age among Iranian girls: Probit analysis on cross-sectional data of Persian (Shahedieh) cohort.

BACKGROUND: Given concerns regarding changing puberty ages globally, examining current trends in specific societies is essential. OBJECTIVE: This study aimed to determine secular changes in the age at menarche among Iranian girls in central Iran using the Probit model. MATERIALS AND METHODS: This cross-sectional study utilized data from the entry phase of the Shahedieh cohort study, Yazd, Iran. In total, fertility information and menarche data were extracted from 4,854 women (aged 35-70 yr). Based on the menarche age data of these individuals, the proportion of those who had reached menarche in each of the 3 birth decades (1946-1957, 1958-1969, and 1970-1981) was calculated. To calculate this proportion, a hypothetical time origin was considered for each decade, and the Probit model was employed to calculate the probability of menarche at each age. RESULTS: Findings indicate that approximately 10% of Iranian girls experienced menarche before age 11, while 90% reached menarche by age 14.89, with a median age of 12.89 yr. A significant downward trend in the age at menarche was observed; girls born between 1946-1957 experienced significantly later menarche compared to those born in the 1958-1969 and 1970-1981 cohorts at the 10 th , 25 th , 50 th , and 75 th percentiles. CONCLUSION: The observed downward trend in menarche age in central Iran aligns with results from similar studies in developing countries. This declining trend has historically been attributed to lifestyle-related variables. Today, this decreasing trend (acceleration of puberty) can be attributed to poor nutritional habits, sedentary lifestyles, or a physiological response to increased physical and mental stress.