The length and quality of a person's menstrual cycle can be a crucial indicator of their overall health, and new research suggests doctors should be monitoring this cycle in adolescence and later adulthood.

A 24-year-long study on nearly 80,000 healthy nurses in the United States has now provided some of the first real evidence on menstruation and premature death.

According to the longitudinal research, which began in 1989, people who experience irregular and long menstrual cycles in adolescence and throughout adulthood are more likely to die before the age of 70 compared to those with shorter and more regular cycles. This link was particularly strong for cardiovascular-related deaths and, to a lesser extent, cancer-related deaths.

"What this study will hopefully achieve is to raise awareness about menstrual irregularity, increase education and encourage women and doctors to consider the menstrual cycle when assessing health," says King's College reproductive physiologist Kim Jonas, who was not involved in the research.

"However, this study does not mean that all women who have experienced irregular menstrual cycles should be concerned. There is a lot more research to be done in this area and many factors are likely to be at play."

Menstruation is sometimes called the 'fifth vital sign' - following temperature, pulse, breathing and blood pressure - and irregular cycles have been linked to a whole bunch of other health factors, including sexual and reproductive disease, bone and heart disease, cancer, mental health problems, and other chronic health conditions.

This doesn't mean irregular menstruation is causing these health effects; it might simply be that whatever causes a longer and more irregular cycle reflects poorer health overall.

Given this association, it's not altogether surprising that irregular menstruation is linked to early death, and yet health care providers often fail to ask about a patient's menstrual cycle when assessing their overall cardiovascular health.

Investigating possible correlations is not easy. Large, reliable datasets on menstruation are hard to come by, and most of these results are based on self-reported surveys or menstrual tracking apps.

The current study is plagued by some of the same limitations, although its length and high follow-up rate with volunteers is impressive.

The research is based on a large cohort of nurses in the United States, aged 25 to 42 years, who were sent mailed or online questionnaires every two years to collect information on their lifestyle, diet, medical history and any disease.

At the start of the study, in 1989, female nurses were asked to recall their menstrual cycles during adolescence (between 14- and 17-years-old) and in early adulthood (between 18- and 22 years-old).

In 1993, the same cohort was asked about the usual length and regularity of their current menstrual cycles, when they were aged between 29 and 46 years.

"We found that the risk of premature mortality was higher among women who reported long or irregular cycles later in life," the authors write.

While this might be due to a diminished recall of previous menstrual cycles in earlier life, it could also be a sign of persistent poor health.

In the older age groups, those who experienced menstruation cycles longer than 40 days were more likely to die prematurely than those who reported a more typical cycle of 26 to 31 days.

This was particularly strong among female nurses who experienced continuous irregular cycles in adolescence and early adulthood, and also those who smoked.

This latter finding makes sense, as smoking is known to impact cardiovascular, immune and metabolic health, and irregular menstruation might be a sign of poor health in these areas.

"This interaction, however, should be interpreted with caution given the marginal statistical significance of the tests," the authors warn.

What's more, most of the nurses in this study were white women of the same profession, and this career requires irregular hours of work, which can impact long-term health and disrupt menstrual regularity.

More research is needed to tease out the associations between menstruation and potentially fatal health conditions, but the new study provides some of the strongest indication to date that irregular menstruation could be linked to poorer health, whether in adolescence or in later adult life.

"These relations were also stronger when long and irregular cycles were consistently present during adolescence and throughout adulthood," the authors write.

Even when other influential factors, like age, weight, lifestyle and family medical history, were taken into account, the results stayed the same, although the authors note they can't be sure they haven't missed out on other contributing factors.

Jacqueline Maybin, a research fellow and gynaecologist at the University of Edinburgh, said the methods of the study were sound and the results important, but for those with irregular periods there's no reason to freak out.

"It is also important to remember that irregular menstruation is a symptom and not a diagnosis. Therefore, a specific underlying cause of irregular menstruation may increase the risk of premature death, rather than the irregular bleeding, per se."

For instance, the authors of the current study speculate that the link between premature death and an irregular cycle might sometimes reflect a disrupted hypothalamic-pituitary-ovarian axis, which is the part of the brain that closely controls female hormones, and is a sign of overall general health.

Given that a random trial is unachievable, the authors of the new study say theirs is the "best available evidence for understanding the long-term health consequences of menstrual cycle characteristics."

Primary care providers should therefore ask patients about their menstrual cycle throughout adolescence and adulthood, as this could be a key factor in assessing their overall health.

"This study should not be a cause of concern for all young women with irregular and/or long menstrual cycles as there are many other factors involved," says maternal scientist Rachel Tribe from Kings College London, who was not involved in the study.

"But I would hope that the information would raise awareness and encourage healthcare providers (as well as women) to investigate irregular menstrual cycles; an approach that has potential to improve reproductive health and subsequent longer term outcomes."

The study was published in BMJ.