Up to 50 million people have downloaded a period or cycle tracking app (CTA), many without realising that all the personal data they punch into it may be sold to third parties for advertising and research purposes.

Now experts have warned that data could be dangerous in the wrong hands and Australian women "have a right to be angry" about it, one expert says.

Used to track menstrual cycles and fertility, CTAs make up more than half of the booming 'femtech' market, which is projected to be worth more than US$60 billion (AU$92.4 billion) by 2027.

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Many Australians use them for convenience and family planning without ever thinking about where their personal data – from period dates to potential pregnancies – goes once they punch it into the app.

Now a report from the University of Cambridge has revealed that CTA data is being sold at scale for profit, calling the apps a "gold mine" for consumer profiling.

User data collected by CTAs can offer insights into the reproductive choices, mental health concerns, sexualities, social habits and politics of individuals and groups – the kind of data many third parties would pay a fortune for.

Have you got a story? Contact reporter Maddison Leach at Google Play.