Natural Cycles — Cycle & Fertility Tracker

Natural Cycles differs from Flo's app already in this category in one specific way: its algorithm has been through published clinical validation studies for both contraceptive and conception use, giving it a stronger evidence basis than most cycle-tracking apps, which typically rely on prediction rather than validated algorithms. It works by combining daily basal temperature readings (via a paired or compatible thermometer) with your cycle data, rather than predicting from calendar data alone.

Against Femometer's standalone Vinca thermometer also in this category, Natural Cycles is the fuller ecosystem — app subscription plus thermometer — versus a standalone hardware device you could pair with any tracking method. The subscription cost is the main trade-off: unlike Flo's free tier, Natural Cycles is subscription-only, reflecting its clinically-validated positioning.