Women who suffer crippling premenstrual stress (PMS) may have an undiagnosed STI, research suggests.
Chlamydia and other bugs double their risk of enduring mood swings, headaches and cramps in the run-up to their period.
Oxford University researchers are unsure as to exactly why STIs can worsen PMS.
But they believe it may be down to a woman’s immune system becoming weaker in the days running up to her period.
This may allow STIs to multiply and thrive, the scientists say, leading to inflammation that could worsen symptoms of PMS.
Women with STIs are twice as likely to endure crippling PMS, such as cramps (stock)
Figures suggest around 80 per cent of fertile women in the UK and US suffer at least one PMS symptom every month.
And up to eight per cent have a severe form of PMS, known as premenstrual dysphoric disorder, which may require treating with antidepressants or prescription painkillers.
The researchers analysed 865 women who were asked via the period-tracking app Clue if they had ever been diagnosed with an STI.
Those who answered ‘yes’ were asked when they were diagnosed and what treatment, if any, they were given.
Clue allows its 10 million users worldwide to report daily data on their menstrual bleeding, PMS symptoms and emotional wellbeing.
The women’s STI diagnoses were then combined with the above data that the participants had previously logged on the app.
All of the participants were also asked about their contraception use, for the study published in the Journal Evolution Medicine & Public Health.
Results suggest women who have an undiagnosed STI are twice as likely to suffer from headache and cramps in the five days leading up to their periods.
STIs also double a woman’s risk of feeling sensitive or sad in the lead up to her period, but do not make her more susceptible to tender breasts.
No evidence was found that hormonal contraceptives, such as the pill, affect a woman’s emotional wellbeing, or risk of cramps or headache.
Hormonal contraceptives may, however, ease a woman’s breast pain and make her feel less sad in the last three days of her cycle.
According to the researchers, PMS is still not taken seriously despite it potentially being a warning sign for women with undiagnosed STIs.
Lead author Dr Alexandra Alvergne said: ‘Even now, when I write a research application on PMS, I still think “will this be taken seriously?” that needs to change.
‘Not understanding or even acknowledging that PMS is more than “women’s raging hormones” but rather, the by-product of cyclical immunity makes it harder to identify diseases and can even delay diagnosis of infections such as STIs, which can affect women’s fertility.’
She added: ‘Our research shows that by better understanding their period and menstrual cycle, women could improve their health.
‘If you know that severe PMS could be an indicator of an underlying STI, you are more likely to listen to your body.’
This comes after previous research suggests women who suffer with severe forms of PMS may be infected with a parasite carried by cats.
Scientists found women with extreme symptoms, such as depression and anger, are more likely to have the parasite toxoplasma gondii in their bloodstream, which can be caught by handling cat litter.