The idea of estimating the fertile window from a menstrual calendar isn’t a recent app invention – it’s nearly a century old. It was developed independently, in the late 1920s, by Austrian physician Hermann Knaus and Japanese gynecologist Kyusaku Ogino.
Working separately, both reached the same scientific conclusion: ovulation can be statistically estimated from the length of previous cycles – the first genuinely systematic approach to fertility calculation.
The Knaus-Ogino method underpinned every later calendar-based fertility approach, from the old “rhythm method” all the way to modern cycle-tracking apps.
A calculation many people now run from an app carries, behind it, almost a century of medical statistics.
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