Abstract:Menarche is the first menstrual period. It is the final event in the physical manifestation of gonadal development. A study in Europe found that the median age of menarche was 12 years and 11 months and that by the tenth year, 0.8% of the girls had their first menses, while the figure for the eleventh and twelfth years were 3.6 and 21.7% respectively. A Geneva study found that menarche was present in 50% at the age of 12.75 years. This study reported that, in the periods 1955-1980, the age of puberty has decreased by 0.5%, while the decrease between the periods 1965-1980 was 0.25%. Studies in the 70s and 80s found that African American girls were taller, heavier and maturing earlier than white girls of their age. Early menarche was observed in some of the girls by the age of 11 years, while all the girls were menstruating at the age of 15 years. Precocious puberty was defined as puberty before the age of eight years, while delayed puberty was defined as the absence of breast budding before the age of 14. However some pediatricians have concluded that the onset of breast development before the ages 7-8 years in white girls and between the ages of 6-7 years in African American girls may be part of the normal variation in the timing of puberty. It had been suspected that the decrease in the age of puberty is due to improved standard of living. However endocrine-disrupting chemicals from environmental pollutants are now seriously implicated as major factors in early puberty, while the ethnic factor has also been recognized. In this study, 489 students aged 15-17 selected from secondary schools from different areas of Trinidad were used. The data was collected through a questionnaire and the primary variable measured was the age of onset of menses. It also contained information on the ethnicity of the respondents. The heights and weights of the students were also measured. Results showed that, the mean age of menarche for Trinidadian girls was 11.71 years ±1.18. It also showed that the mean age for Blacks was 11.67±1.29, while that of Indians was 11.65±1.08. The mixed races' mean, was 11.87±1.09. No significant difference was found in the mean age of menarche between Blacks and East Indians. The result also showed a decrease in the age of menarche with increased BMI. The mean age of menarche of the mothers of the students sampled was 12.57 years, which is significantly higher that that of their daughters. This is consistent with recent findings, which reported that the mean age of menarche has been decreasing over the years in all societies.