ANNA ROTHSCHILD: This is Science Friday. I’m Anna Rothschild.

There’s a growing body of research that’s revealing something strange about puberty. Kids these days are getting their periods earlier than previous generations did, and those periods are less regular for this younger generation too.

A recent study from Harvard’s School of Public Health has uncovered another strange twist to this finding. The earliest and most irregular periods happen among racial-minority and low-income families. This is a big deal because early menarche, someone’s first menstrual period, can be a sign of future health issues.

Joining me to help break this down is the study’s lead author, Dr. Zifan Wang, postdoctoral research fellow at Harvard in Cambridge, Massachusetts. Welcome to Science Friday.

ZIFAN WANG: Hi. Thank you for having me.

ANNA ROTHSCHILD: So let’s start with your findings from this study. Can you just explain what you found?

ZIFAN WANG: Yeah, sure. So we looked at individuals within the US who were born between 1950 to 2005, and we found that younger generations tend to start their first period, or what we call menarche, at a younger age. So, on average, that’s from 12.5 years old for those who are born in the 1950s all the way down to 11.9 years old among those who were born around 2005. And we also saw that the rate of early menarche, which is defined as first period before 11 years old, has also doubled over the years from 9% to about 16%.

One other thing that we did find very interesting is another marker for menstrual health, which is the time it takes for someone to establish regular menstrual cycles since their first period. This time is also becoming longer for younger generations. And just as you mentioned, we also saw these trends to be more heavily impacting people of color, including Black, Hispanic, and Asian individuals when compared to their white peers.

ANNA ROTHSCHILD: Why is it important to track irregular periods specifically?

ZIFAN WANG: So in other literature, researchers have found that either early menarche or irregular periods to be correlated with disease later in life, such as cardiovascular disease, certain types of cancer, as well as infertility. So the trends that we’re seeing in terms of declining age and menarche as well as increasing menstrual cycle irregularity are a bit concerning due to that reason since it’s really heavily connected to later-life health conditions.

ANNA ROTHSCHILD: You talked about how this study is looking at a wide range of people, people born between 1950 and 2005. Can you explain a little bit how you collected the data for this study?

ZIFAN WANG: Yes, absolutely. So our analysis used self-reported data from surveys among participants from the Apple Women’s Health Study, which is a US-based digital cohort study. This study is a collaboration between Harvard, Apple, and the National Institute of Environmental Health Sciences. And those who were included in our analysis met the eligibility criteria of having an iPhone, being at the age of consent, living in the US, and have menstruated at least once in their life.

And we analyzed these trends based on participants’ self-reported age of menarche as well as their self-reported time from menarche to establishing regular cycles, and we plotted these trends over birth-year categories, and that’s where we found the declining trend as well as disparities across different self-identified race and ethnicity groups.

ANNA ROTHSCHILD: Got you. I have a question for you. Especially among older generations, how reliable is the information about period regularity? I remember how old I was when I got my period, but I can’t really remember how old I was when my period got to be regular. That’s not really something I was tracking, particularly before iPhones existed.

ZIFAN WANG: Yeah, I would say there’s still a gap in research in just studying menstrual health, especially menstrual health during adolescence in general. There have been some other studies documenting that the reliability of self-reported age at menarche is actually pretty high, but we are not sure whether self-reported time from menarche to regularity are also kind of as reliable as age in menarche. So this is something that we need to look into the future.

ANNA ROTHSCHILD: Right. That makes sense. So based on the information that you’ve collected so far, do you expect this trend of early menarche to continue into the future?

ZIFAN WANG: Yeah, so this declining trend is actually not super new because other studies have also documented in earlier years that this trend is happening, and we’re seeing also among the generations born between 2000 to 2005, and this is continuing. So I think there’s reason to believe that this trend will continue unless we work on the tangible solutions that might be contributing to this trend and disparities.

ANNA ROTHSCHILD: How might starting their period earlier impact a kid’s long-term physical and mental health?

ZIFAN WANG: Yeah, this is a good question. So a part of the reason that we are trying to document this trend is really trying to make parents and clinicians aware that we should be more prepared for children experiencing their first period at an earlier age so that people can initiate proactive discussions around menstruation early on. Like from the educational standpoint, this could really help foster the understanding and reduce the feelings of shame or embarrassment that is often tied to their first period and just recognizing menstrual health as a vital sign.

ANNA ROTHSCHILD: Great. Are there physical effects of starting periods earlier as well though? Like, are there sort of long-term health effects?

ZIFAN WANG: Yeah, some of the long-term health effects that I mentioned earlier are just increased risk of metabolic conditions like diabetes, high cholesterol, and, in the end, cardiovascular disease. Earlier menarche and irregular cycles are also associated with higher risk of infertility or developing breast cancers later in life.

ANNA ROTHSCHILD: Got you. So it’s that it’s both starting periods earlier and irregular cycles that both contribute to those things, but we don’t have, necessarily, more granular data about which of those is contributing to what.

ZIFAN WANG: Yeah. So other researchers have separately looked at how earlier menarche plays a role in these disease conditions, and then other groups of researchers have also separately looked at how longer time to regularity might also play a role. But at this point, it’s still unclear whether they are jointly impacting the outcomes or is that like a sequential step because there’s definitely a time element around these measures.

ANNA ROTHSCHILD: So a study from last year found that Black kids are more than twice as likely as white kids to start puberty early, which is just so staggering. And now your study shows that Black, Hispanic, and Asian kids all tend to start their periods earlier than white kids. Do we know why this is happening?

ZIFAN WANG: Yeah, this is very concerning, and there are multiple possible explanations for this, but it’s hard to really just pinpoint to one factor. In our analysis, we looked at how obesity might play a role. So we looked at, again, participants’ self-reported body mass index around the time of their first period, and we found that BMI at first period might be explaining about 46% of the trends that we’re seeing in age at menarche. But we’re still seeing remaining trends even after we adjusted for BMI in our analysis.

So there could be some other possible players based on other researchers’ work, things like air pollution, toxic chemicals from the environment, or limited access to healthy food, all of which are also disproportionately impacting people of color. Unfortunately in our analysis, we are limited in our data to really further investigate these factors, so we definitely need more research on this area as a next step to work on the more modifiable or tangible solutions to these trends and disparities.

ANNA ROTHSCHILD: We know that a lot of socioeconomic factors are connected to a person’s overall health. Like access to health insurance or if you live in a polluted area, those things will have a big impact on how healthy a person is. Do we know how much of the results of this study are correlation versus causation?

ZIFAN WANG: So for this study, we are really just trying to document the trends that we’re seeing. We didn’t really adjust for any what we call potential confounding factors. So our goal for this initial study is really just to show the distribution and show the disparities across different disadvantaged socioeconomic groups. And in the future, we definitely want to do more granular estimates of, Is there a factor that’s causing this trend? versus is it really just a correlation?

ANNA ROTHSCHILD: This is just sort of a basic question, but how well researched is menstruation? It’s something that roughly 50% of the population does in their lives at some point, but how much do scientists really know about it?

ZIFAN WANG: I’m so glad you brought this up, and it’s interesting because a few years ago, our study team just looked up the search term of menstruation in the PubMed database, and we kind of looked at the period between 2001 to 2018 in PubMed, and there were only around like 8,000 publications relevant to menstruation. And in comparison, there were more than 900,000 publications on cardiovascular disease, more than 100,000 on prostate cancer, and, interestingly, more than 16,000 on erectile dysfunction.

And we know that menstruation is impacting, as you just said, around half of the population. So there’s definitely a gap there in terms of research in the menstrual cycle in general. Lately, including the Apple Women’s Health Study, there have been like this emerge of digital app-based studies taken from menstrual-tracking apps, for example. So hopefully we are on our way to work towards addressing these gaps.

ANNA ROTHSCHILD: That’s great. Do you think that it’s just a matter of representation of who’s doing research, or do you think that there’s more urgency for other reasons and that’s why we’re now seeing more research on menstruation in recent years?

ZIFAN WANG: Yeah, we haven’t really dived into the reasons behind this gap. But in general, I feel that in more recent years, there has been more awareness amongst the public as well as among clinicians to recognize menstrual cycle as an important vital sign. So yeah, I think we’re making progress here.

ANNA ROTHSCHILD: Were you able to find past research from decades ago that you could compare with your results?

ZIFAN WANG: Yes. There are multiple publications either within the US or in other developed countries that are also showing similar trends in terms of decreased age at menarche. I will say that most of these studies, which were taken from an earlier time period, when they studied disparities, they mostly just focused on Black versus white disparities based on their data availability. So one of the novel things we found in this data is that, first of all, this is impacting all race and ethnicity groups. And second, this is not just impacting the Black individuals but also Hispanics and Asian individuals as well.

ANNA ROTHSCHILD: This is having an impact on a lot of people, as your study has found. If someone is listening to this conversation and either they or their kids had an early period or an irregular period and they’re worried about the potential health effects, what did you recommend that they do?

ZIFAN WANG: Yeah, that’s a good question. And speaking as someone who wish to become a parent too, I think this does raise some alarms. And as a society, I think we should focus on greater investments in the early counseling, education around menstrual health, as well as individualized health-care plans early on during the life course.

Individually, a healthy lifestyle in general would be helpful, like physical activity and healthier diet. In our team, we did a separate research that looked at the association between irregular cycles and cardiometabolic conditions, and we found that physical activity and BMI might modify the risk of cardiovascular disease associated with irregular cycles. So there are definitely some ways to kind of mitigate the risk, even if you already had your first period.

ANNA ROTHSCHILD: OK, that’s great to know. There’s obviously a lot of information in this study that will be so important to understand further. What are you most interested in studying in the future?

ZIFAN WANG: Yeah, our team is really dedicated to documenting the factors around menstruation and also menstrual hygiene. For an overall goal, just to first reduce stigma around this topic, and as subsequent steps, we are utilizing the knowledge and methods that we have to identify the modifiable factors to reduce risk across the lifespan.

ANNA ROTHSCHILD: Well, that’s all the time we have for now. I’d like to thank my guest, Dr. Zifan Wang, postdoctoral research fellow at Harvard in Cambridge, Massachusetts. Thanks for joining us.

ZIFAN WANG: Thank you.

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