Trauma-related symptoms may be more or less intense at different points in menstrual cycles, posing challenges for women who have experienced trauma and doctors diagnosing them if their condition amounts to post-traumatic stress disorder, or PTSD.

A study of 40 naturally cycling women with a history of trauma suggests people may experience more symptoms during the first few days of the cycle when the hormone estradiol is low, and fewer symptoms close to ovulation, when estradiol is high.

The results could have implications for how PTSD is diagnosed and treated among women and people with periods.

"When in the cycle you assess women might actually affect whether they meet diagnostic criteria for PTSD, especially for people who are right on the border," says psychologist Jenna Rieder of Thomas Jefferson University, Philadelphia.

Understanding how moods and symptoms are influenced by sex hormones could also help women – who are twice as likely as men to develop develop PTSD after trauma but have long been neglected in research – manage their experiences post-trauma.

"For women who are naturally cycling, it may be useful to understand how the menstrual cycle affects their symptoms. When you can explain what's happening biologically, it often becomes less threatening," says Rieder, who led the study.

PTSD, which used to be known as 'shell shock' or combat fatigue, is a type of anxiety disorder that can develop after someone experiences or witnesses a traumatic event such as a serious accident, violence, natural disasters, or physical or sexual assault.

An estimated 1 in 11 people will be diagnosed with PTSD in their lifetime. Symptoms include vivid memories, intrusive thoughts, flashbacks or nightmares, as well as emotional numbness and avoidance behaviors, all of which can be triggered in day-to-day life by cues that remind someone of the original trauma.

It's thought that fluctuations in sex hormones, namely estradiol – a type of estrogen – might contribute to women's susceptibility to developing trauma-related stress and PTSD.

Estradiol is the most biologically potent type of estrogen hormone in women who are not pregnant or menopausal; rising estradiol across their menstrual cycle ultimately triggers ovulation.

Research has also linked low levels of estradiol, which occurs early on in the menstrual cycle, with stress, fear, and greater activity in the emotional centers of the brain.

But untangling the effect of estradiol from other sex hormones that also fluctuate throughout the menstrual cycle, including progesterone, is difficult. Results from some past studies attempting to understand the influence of sex hormones on PTSD symptoms have been muddled by sampling issues.

In this study, the researchers not only looked at distinct phases of the menstrual cycle where hormone levels differ substantially, they also measured estradiol levels and two stress biomarkers in women's saliva to make the connection between sex hormones, stress response, and trauma symptoms. Women with irregular periods were also excluded from the study.

Among a group of 40 women asked to recount the traumatic event they had experienced or witnessed, those who had lower estradiol levels reported having more severe avoidance symptoms in the past month; they also had an abnormal stress profile.

A smaller group of 30 women then monitored their mood and the severity of their symptoms 5 times a day over 10 days of their menstrual cycle, starting 2 days after bleeding, to tease out the relationship between estradiol levels and trauma-related symptoms over time.

People's moods varied more on low-estradiol days of their cycle. They also reported more severe PTSD symptoms on those days, driven by low moods, inappropriate self-blame and exaggerated negative beliefs, the study found.

"Our results suggest that lower estradiol might render women more prone to negative affective states [moods] and thus negative thoughts or feelings about past events, including past trauma," Rieder and colleagues write.

Based on these findings, Rieder and colleagues suggest doctors could factor menstrual cycles into their care plans for people with PTSD, in an effort to improve therapy outcomes. They could also help someone anticipate changes in trauma-related symptoms that may occur month to month.

"Low-estradiol portions of the cycle might be the time window where interviews would be most sensitive to any current PTSD symptoms and when the reported symptoms might be most severe," Rieder and colleagues add.

It's important to note, however, that the women in this study had low-to-moderate symptoms and were not necessarily diagnosed with PTSD. So the study ideally needs to be carefully replicated in women with PTSD, to see if the findings hold true.

Menstrual cycle-driven fluctuations in mood and symptoms may be more evident in people diagnosed with PTSD, but "it is also possible that women with PTSD experience symptoms more consistently across the cycle, with no alleviation of symptoms during high estradiol phases," the study authors conclude.

The research was published in the journal Psychological Trauma: Theory, Research, Practice, and Policy.