What Natural Cycles Does
Natural Cycles uses your daily basal body temperature (taken with an oral thermometer each morning) plus optional LH test results to algorithmically predict your fertile window and confirm ovulation. The app classifies each day as "green" (not fertile) or "red" (fertile/potentially fertile). For TTC users, red days are your signal to have sex.
The app is FDA-cleared (510(k) clearance as a contraceptive device, with a separate pregnancy planning mode). It uses a sophisticated algorithm that learns your individual cycle patterns over time, becoming more accurate with each cycle of data.
Pros
- Regulatory clearance — it's the only fertility app with FDA 510(k) clearance, which means it's been evaluated for accuracy claims
- Algorithm improves over time — the more data you input, the more personalized predictions become
- Integrates LH tests — you can log OPK results to improve prediction accuracy
- Works with wearables — compatible with Oura Ring and Apple Watch for automatic temperature input
- Pregnancy mode — switches to pregnancy tracking after conception with fetal development info
- Clean, modern interface — easy to use, charts are intuitive
Cons
- Requires consistent morning temping — you need to take your temperature at roughly the same time each morning before getting out of bed (unless using a wearable)
- BBT confirms, it doesn't predict — temperature rises after ovulation, so the app relies on historical patterns to predict future fertile windows. First 1-2 cycles have more "red" (uncertain) days
- Subscription model — $99.99/year or $12.99/month (includes a basal thermometer with annual plan)
- Less accurate with irregular cycles — women with PMOS or very irregular cycles get more red days and less precise predictions
- Temperature is a lagging indicator — OPK strips detect the LH surge before ovulation, giving you advance notice. BBT only confirms after the fact
Natural Cycles is a solid tool, but it works best as part of a tracking system, not a replacement for OPKs. The algorithm confirms ovulation well, but for TTC purposes, you want a predictive signal (OPK or cervical mucus) plus a confirmatory signal (BBT/Natural Cycles). Using both gives you the full picture.
Natural Cycles vs Alternatives
- vs OPK strips alone: OPKs predict ovulation 12-36 hours in advance (better for timing sex). Natural Cycles confirms it happened (better for understanding your cycle). Ideally, use both.
- vs Tempdrop: Tempdrop tracks BBT automatically while you sleep — no morning routine needed. If you find daily oral temping unsustainable, Tempdrop + a charting app may be a better fit than Natural Cycles.
- vs free apps (Flo, Clue): Free apps predict fertility based on cycle length averages — not your actual temperature data. Natural Cycles uses real physiological data, which is significantly more accurate.
Is It Worth $100/Year for TTC?
If you're committed to daily oral temping and want an algorithm to interpret the data for you, yes — it's a reasonable investment. The included thermometer (with annual plan) is a nice touch. But if you already use a wearable like Tempdrop and chart with its free app, Natural Cycles adds limited value.
For most TTC women, the highest-ROI approach is: cheap OPK strips ($8 for 50) + cervical mucus tracking + a BBT thermometer. Total cost: under $20. It's not as polished as Natural Cycles, but it gives you the same data.