Young girls are starting their first periods earlier than they have in previous decades—a shift associated with adverse health outcomes later in life.A new study published on May 29 in JAMA Network Open revealed that the median age at menarche has remained relatively stable at around 12 years, and the proportion of girls starting menstruation before age 11 has significantly increased over time.Menarche, or the first menstrual period, marks the beginning of the monthly hormonal cycle and reproductive lifespan. Additionally, it signifies the end of female puberty.Researchers with the Harvard T.H. Chan School of Public Health’s Apple Women’s Health Study...