There are thought to have been as many as 200m downloads of period tracking apps worldwide.
But a leading UK medical royal college has warned that women should not rely on them as a form of contraception.
And privacy campaigners say users should be wary of what data they might unwittingly be sharing when they download some apps.
A search for period tracking apps will quickly bring up dozens of offerings - some with charts, others illustrated with pink tulips or lilac flowers.
