The only certified and clinically proven contraceptive app

Based on a unique algorithm, the app analyses fluctuations in a woman’s basal body temperature to identify ovulation and predict fertile days. Women use the app in combination with a basal thermometer to identify fertile days when protection (or abstinence) is required. Natural Cycles is the only medical app that has been certified in the EU as a Medical Device for use in Contraception.

Contraceptive reliability

The Pearl Index of Natural Cycles for typical use (7.0) and perfect use (1.0) has been demonstrated in two separate prospective, observational effectiveness studies, based on real-world data. Both studies were published in peer-reviewed journals (see our Publication and Clinical Findings).

“Natural Cycles identifies the fertile window for each individual woman based on her unique cycle pattern, changes in hormone levels and temperature fluctuations.”

How does Natural Cycles work?

Women are only at risk of pregnancy during their fertile window. This is the day of ovulation and taking sperm survival in the body into account, the five preceding days.¹ Without knowing when these 6 days are, contraception is needed throughout the cycle.

Natural Cycles identifies the fertile window for each individual woman, based on her unique cycle pattern, changes in hormone levels and temperature fluctuations. Established biological indicators of ovulation include:

  • A temperature rise of 0.2 – 0.45°C (0.54 °F), when progesterone is released
  • A surge in luteinising hormone (LH) approx. 48 hours prior to ovulation

The Natural Cycles unique algorithm analyses a woman’s basal body temperature (BBT) and identifies when ovulation has occurred and the woman is no longer fertile. By measuring the basal temperature in the morning (5 values per week are recommended) and entering the data into the app it gets to know the woman and her individual menstrual cycle. After 1-3 cycles of measuring, the app has enough data to confirm when ovulation has occurred and accurately predict the fertile window.

Women can also supplement the BBT data by measuring their Luteinising Hormone (LH) levels by means of a simple urine test strip and entering this data into the app.

The image above illustrates the increase in temperature that occurs after ovulation, and how the hormone levels influence the basal temperature.

The algorithm

Natural Cycles is based on a sophisticated bio-statistic algorithm that takes a woman’s temperature and many other factors such as sperm survival, temperature fluctuations and cycle irregularities into account. It detects ovulation, fertility and the different stages of her cycle, and also calculates accurate predictions for upcoming cycles. It has been clinically proven to be highly accurate in detecting and predicting ovulation.

A woman measures her temperature under the tongue every morning, enters the temperature, as well as her menstruation data into the app. A sophisticated bio-statistic algorithm analyses these data and informs the woman if she has a “red day” – when there is a risk of pregnancy – or a “green day”. On red days, women are required to use protection (or abstain from sex) in order to prevent a pregnancy.


¹Wilcox AJ et al. N Engl J Med. 1995; 333: 1517-1521