AMNA NAWAZ: A new study this week adds to the growing body of evidence that girls in America are reaching puberty earlier, with potentially troubling implications for their long-term health.

More girls are getting their first periods sooner than previous generations and experiencing longer periods of menstrual irregularity as well.

We're joined now by the senior author of that study, Dr. Shruthi Mahalingaiah, assistant professor at the Harvard T.H.

Chan School of Public Health.

Doctor, welcome, and thanks for joining us.

DR. SHRUTHI MAHALINGAIAH, Harvard T.H.

Chan School of Public Health: Thank you.

AMNA NAWAZ: So let me just tick through some highlights of your study real quick here.

You found there that girls are now -- or, rather, were getting their first period on average at the age of 12.5 between 1950 and 1969.

Now, between 2000 and 2005, that's down to 11.9 years old.

You also found the percentage of girls getting their period before the age of 11 almost doubled from about 8.5 to 15.5, and the percentage of girls getting their period before the age of 9 more than doubled.

Doctor, why is this significant and worth paying attention to?

DR. SHRUTHI MAHALINGAIAH: Sure.

The age at the first period and time to cycle regularity are two aspects of menstrual health.

It is really, really important to understand this trend for a variety of reasons.

One of the most important reasons is that both of these characteristics are associated with a variety of long-term health consequences.

And for those with very early age at the first period or menarche and persistently irregular periods, those risks include risk of cardiovascular disease, infertility or fertility problems, mood disorders, and cancers, to name a few.

AMNA NAWAZ: And the big question here, of course, is, why?

Why are we seeing this trend?

What do we know about that?

DR. SHRUTHI MAHALINGAIAH: When we evaluated that exact question, we thought about, how could body mass index, particularly around the age at the first period, influence this.

And we found that about half of this trend could be explained by body mass index at menarche.

When we looked at maybe what else might be going on, controlling for body mass index, we still noticed a trend towards decreasing age at the first period.

And there are a variety of concerns I have around this and other studies have shown, including exposure to environmental factors, like hormone-disrupting chemicals, air pollutants, heavy metals, and exposures to stress and childhood trauma are some important things to consider.

AMNA NAWAZ: There were also some substantial differences along racial and socioeconomic lines, girls of color, in particular, starting menstruation earlier relative to their white peers.

What explains that?

DR. SHRUTHI MAHALINGAIAH: When we looked at the absolute difference, comparing those of white race ethnicity to other racial minorities, the kind of white girls reduced that age at first period to about six months, and other race ethnic groups had a reduction of almost a year from the first birth cohort to the most recent.

We think about race ethnicity as a surrogate marker for exposures.

So, we're thinking about what are the other exposures that might covary with being in a particular race ethnic group, from exposures related to personal care products to stress and racism.

AMNA NAWAZ: You know, it's worth pointing out that this study was based on self-reported data, right?

It was through an app.

Over 70,000 women participated.

What should we understand about the limitations of data collected that way and what questions you still have?

DR. SHRUTHI MAHALINGAIAH: There are many important limitations to consider about data collected by self-report.

And there are limitations regarding how we collected the data.

Participants had to have an iPhone and download the research app and be comfortable communicating in English.

So we do have to interpret all of this information in the context of the population recruited into the study.

However, I think that what we are finding does allow a high-level look into the time trends across a vast time span of birth cohorts.

And we are very much interested in promoting awareness of this topic to allow for education of providers and children.

And in terms of future research, looking into further impacts of environmental exposures is something that I'm very interested in.

AMNA NAWAZ: So, to that goal of raising awareness here, a lot of parents are going to hear this.

They're going to be very worried.

They're going to wonder what they should do, if anything, to keep their children safe and healthy.

What would you say to them?

DR. SHRUTHI MAHALINGAIAH: For parents and caregivers, if you notice early signs of puberty, like breast development, going into the pediatrician for an evaluation is really important.

And then, in terms of what you can do to promote health and wellness around the menstrual cycle, include a diet of whole foods.

It's very hard in today's modern world to have a diet of no ultra-processed foods, but try to limit processed foods and fast foods as much as possible and bring in those leafy green vegetables, fruits is really important.

Addressing other aspects of health through behavior, including balancing physical activity and addressing circadian rhythms and sleep health is very important in promoting health in this age group.

AMNA NAWAZ: That is Dr. Shruthi Mahalingaiah, assistant professor at the Harvard T.H.

Chan School of Public Health.

Doctor, thank you so much for joining us.

We appreciate your time.

DR. SHRUTHI MAHALINGAIAH: Thank you.

OK, take care.