Girls in the U.S. are getting their first periods earlier in life than those in the past generations, and are taking longer time to have regularized cycles, a large-scale study revealed.

According to the study published in the journal JAMA Network Open, the trend toward earlier menarche and longer time for menstrual regularity is more pronounced among Black, Hispanic, Asian, and mixed-race participants.

The researchers considered it as early menstruation when the first periods occurred before age 11, very early when it occurred before the age of 9, and late when the periods started after the age of 16.

After analyzing data from more than 71,000 participants born between 1950 and 2005, the researchers noted that girls born between 2000 and 2005 had menarche at an average age of 11.9 years, almost six months earlier than those born between 1950 and 1969.

The study found that with an increase in the birth year, the average age of a girl's first period decreased.

"In this cohort study of 71 341 individuals in the US, as birth year increased, mean age at menarche decreased and time to regularity increased. The trends were stronger among racial and ethnic minority groups and individuals of low self-rated socioeconomic status. These trends may contribute to the increase in adverse health outcomes and disparities in the US," the researchers wrote.

Earlier studies have shown that environmental factors such as endocrine-disrupting chemicals, metals, or air pollutants, dietary patterns, psychosocial stress, and adverse childhood experiences can all affect the timing of puberty.

The researchers of the latest study believe that increasing obesity among children in the U.S. could be a factor that contributes to early menarche.

"In our exploratory analysis, we found that BMI at menarche may explain 46% of the temporal trends in menarche. This finding suggests that childhood obesity, a risk factor for earlier puberty, which has increased in the US, could be a contributing factor to the trend toward earlier menarche. However, the remaining 54% remain unclear," the researchers wrote.

Girls attaining early puberty have early fusion of the epiphyseal growth plates, resulting in reduced adult height than their potential genetic height. Early menarche is also associated with an increased risk of metabolic syndrome, type 2 diabetes, cardiovascular diseases, and breast cancer.

Read more