What Is Menarche?

What is menarche? There are two menarche definitions are:

1. A girl's first period
2. The period of a girl's life during which she gets her first period.

Unlike later years, menarche is defined by unpredictable bleeding, potentially heavy cramps and other PMS symptoms, and other signs of adolescence. In the U.S., girls typically get their first period between 12 and 13 years old, though it can come on earlier or later depending on a variety of factors.

In many cultures, menarche isn't just a physiological change: It can be considered the main event of adolescence or emblematic of the transition to adulthood.

For girls, it can be a challenging time that they don't fully understand. And for their parents, explaining what it is and why it matters isn't always easy. Whether you just got your first period, need a working menarche definition for young people in your life, or want to learn more about female health, here are the answers to x essential questions.

Menarche Definition: A girl's first period (and more broadly the time of her life during which she has her first period).

More scientifically, it's the first instance of a girl or woman's period: the monthly shedding of her uterine lining without fertilization. Unlike later menstrual cycles, which are more regular (every 21-40 days, on average), menarche periods are notoriously irregular and may come with uncomfortable cramps or irregular bleeding.

Disclaimer: We are not doctors. None of this content is medical advice and was written for informational purposes only. If you are concerned about your or your child's health, please speak with your physician. 

What Is the Average Age of Menarche?

In the U.S., girls get their first period at age 11.9 years old, according to CDC data. By 14 years old, 90% of American girls have had their first period.

Globally, the average age of menarche is 13 years old.

Interestingly, the average age at which a girl has her first period has steadily declined. In 1995, the average American girl got her first period at 12.1 years old; from 2013-2017, the average age went down to 11.9 years old.

There are many theories for why this is, ranging from physiological and psychological.

Related Reading: "How Long Does a Period Last?"

What Factors Affect the Average Age of Menarche?

There are many factors that affect when a girl gets her first period, including body weight, wealth, race, nutrition, and life experience.

1. Bodyweight: A BMI (body mass index) of 17 is typically necessary to menstruate. Overweight and obese children are more likely to experience early menarche, according to research.
2. Wealth: Girls from higher socio-economic backgrounds and those who live with both parents are less likely to experience early periods.
3. Race: African-American girls have theirs, on average, 3 months earlier than white girls. In a 20-year study, the average age of menarche decreased by 9.5 months in African American girls, compared to 2 months in white children.
4. Nutrition: Not being breast-fed and poor nutrition may affect the average age.
5. Health: Children who have a low birth weight, exercised little as children, or were exposed to smoking are more likely to experience menstruation early.
6. Life experiences: Children who have been sexually abused, come from families with significant levels of conflict or have high levels of stress are more likely to experience early menarche.

Why Do African-American Girls Get Their Periods Earlier on Average?

There are several answers to this question, some of which is contradictory. Some research suggests that black children are taller and heavier than white children of the same age. As a result, they experience sexual development first.

By contrast, Mexican-American children experience menarche earlier than white children by a less significant margin, according to research.

Others argue that socio-economic factors account for the difference in age: African American children experiencing high levels of poverty--which translates to poorer nutrition, more stress at home, and a higher chance of obesity.

Is There a Correlation Between First Sexual Experience and Early Menarche?

Interestingly, there is a linear correlation between first period and first sexual encounter. According to CDC data, the older a girl is when she gets her first period, the older she is when she has sex for the first time.