Oct. 9, 2002 -- Today, American girls begin menstruating more than nine months earlier, on average, than they did 20 years ago. Researchers say it's a continuation of a trend that began in the 19th century, but what's more puzzling is the growing gap between black girls and white girls.
While the average age of a girl's first period still hovers around 12, a new study shows black girls start menstruating an average of three months earlier than white girls. And the average age that menstruation began dropped by more than nine months for black girls, but by only two months among white girls between 1973 and 1994.
The findings appear in the October issue of Pediatrics.
In the study, researchers looked at whether the differences between white and black girls' ages at first menstruation were due to factors such as weight, height, or skin-fold thickness (a measure of body fat). But even after adjusting for these differences, they found that black girls were 40% more likely than white girls to begin menstruation before age 11.
About 10% of white girls and 15% of black girls in the study had started menstruating before age 11, which is considered early menarche. Early menarche has been linked to an increased risk of breast cancer, obesity, and miscarriage.
Researchers also found that the girls who'd started their periods before age 11 were heavier and taller than girls who'd started after age 13. In addition, they found 5- to 9-year-old black girls were taller and weighed more than white girls of the same age.
The study authors say differences in height and weight may indicate that black girls were more advanced in terms of skeletal development than white girls. But when they compared black and white girls of similar age, weight, and stature, they found that black girls still started menstruating earlier than white girls.
Study author David S. Freeman, PhD, of the Division of Nutrition and Physical Activity at the CDC, and colleagues says the findings suggest that the annual change in height might be a better predictor of when a girl will start menstruation than actual height, but it did not help explain the racial differences.
The authors say more studies are needed to find out what factors influence when a girl begins menstruating and explore possible explanations for the differences between black and white girls.
Researchers gathered their data from examinations of more than 5,000 girls and follow-up information on about 2,000 of these girls who were examined two or more times over an average of about 6 years.