Intimate data from a range of period-tracking apps is being shared with Facebook, according to a study by Privacy International (PI).
Shared data included details such as what contraception was used, when periods were due and the type of symptoms experienced.
Menstruation apps collect data about women’s general health, information about sex, moods, what the user eats, drinks and sometimes what sanitary products she uses.
The app then uses this information to ‘calculate’ the dates of the month she is most fertile or when to expect her next period.
Facebook’s software development kit (SDK) comprises of tools that can be used by such apps to help them make money by reaching advertisers who, in turn, provide users with personalised ads.
PI found the most popular apps in this category - Period Tracker, Period Track Flo and Clue Period Tracker did not share data with Facebook.
But others - such as Maya by Plackal Tech (which has give million downloads on Google Play), MIA by Mobapp Development Limited (one million downloads) and My Period Tracker by Linchpin Health (more than one million downloads) - did.
Since the investigation, only the Maya app has said it was changing its privacy policies with immediate effect. Linchpin Health did not respond to PI or the BBC when asked for comment, and MIA said it did not want its response to be published.
PI believes its findings raise serious concerns as to how such apps are compliant with the EU's General Data Protection Regulation.
Facebook told the BBC: "Our terms of service prohibit developers from sending us sensitive health information and we enforce against them when we learn they are. In addition, ad targeting based on people's interests does not leverage information gleaned from people's activity across other apps or websites."
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Maria Rafferty is a Belfast-based acupuncturist, herbalist and women’s health coach. She has treated clients with fertility problems, endometriosis and period problems for 10 years.
She told Sync NI: “Not only are these apps sharing women's data, but the information they are giving women based on such data may be inaccurate anyway. For example, an app might tell a woman that she is ovulating on a Monday, but she has no way of knowing for sure and it could be wrong. The apps are based on general algorithms and are not connected to the individual user’s body. They don’t take into account illness, holidays, long haul flights, stress or change in sleeping habits or diet. All of these things impact our hormones and can therefore change the regularity of our fertile window.”
She encourages women to learn how to track their own menstrual cycles by recognising the fertile signs of the body, which she says are obvious once explained: “Being able to tell when you’re fertile is actually very easy. Many women don't realise that their fertile window is only five to six days. Many believe that they can get pregnant at any stage of the month. All a woman really needs is a pen and paper to make a note. Within two to three months she will have found her own rhythm and will not only know her own menstrual cycle but also when she is fertile."
Tracking an individual's menstrual cycle is not only about monitoring periods and fertile times, which is the main purpose of these apps.
Maria added: "Women's energy levels vary throughout the menstrual cycle. Around ovulation time a female will have more energy, feel more creative and be more enthusiastic. However closer to her period she might feel the opposite of this, and may become more lethargic. Once a woman understands her own rhythm, she can make plans round this to take advantage of her hormones.
"Furthermore if a woman knows her cycle and it deviates from her normal time, she will know that there may be an underlying health problem brewing and she can thus seek appropriate medical advice."
PI concluded on the matter, saying: "The responsibility should be on the companies to comply with their legal obligations and live up to the trust that users have placed in them when deciding to use their service."
Facebook recently announced a tool which will further allow users to stop other apps and businesses sharing their data to the social network.
