Natural Cycles is a digital health product for monitoring fertility, used to either prevent or plan a pregnancy. The method tracks physiological signals to determine daily fertility status, providing users with information about their individual menstrual cycles.
Understanding Menstrual Cycle Hormones and Temperature
The menstrual cycle is governed by an interplay of hormones that cause predictable physiological changes. It is divided into two main phases: the follicular phase and the luteal phase, separated by ovulation. During the follicular phase, Follicle-Stimulating Hormone (FSH) encourages follicles in the ovary to mature. These developing follicles produce estrogen, which thickens the uterine lining.
A surge of Luteinizing Hormone (LH) triggers ovulation, the release of an egg from the most mature follicle. Following ovulation, the cycle enters the luteal phase where the remnant of the follicle, the corpus luteum, produces progesterone. This rise in progesterone prepares the uterus for pregnancy and causes a slight increase in a person’s Basal Body Temperature (BBT).
This temperature elevation, between 0.5 and 1 degree Fahrenheit, is sustained for the remainder of the luteal phase. If pregnancy does not occur, the corpus luteum breaks down, leading to a drop in progesterone and estrogen. This hormonal decline causes the uterine lining to shed, initiating menstruation and a new cycle, which returns body temperature to its lower baseline. The link between progesterone and the BBT shift is the primary biological sign used by these methods.
How Natural Cycles Interprets Your Body’s Data
Natural Cycles uses an algorithm to analyze user data and identify menstrual cycle patterns. The primary data point is daily Basal Body Temperature (BBT), taken immediately upon waking. Users measure their temperature, often orally, and input the reading and menstruation dates into the application.
The algorithm detects the sustained temperature increase confirming ovulation, then identifies daily fertility status as a “red day” (fertile) or “green day” (not fertile). On red days, users should use protection or abstain from intercourse to prevent pregnancy. The system’s predictions personalize over time as it accumulates more data from an individual’s cycles.
The application can also incorporate optional Luteinizing Hormone (LH) test results. These tests detect the LH surge that precedes ovulation, providing additional data for the algorithm. This information helps refine the fertile window prediction.
Scientific Validation of Natural Cycles
Contraceptive effectiveness is measured by distinguishing between “typical use,” which includes user error, and “perfect use,” where rules are followed exactly. Natural Cycles is 93% effective with typical use and 98% effective with perfect use.
The application’s performance as a contraceptive is supported by clinical studies, and this evidence was part of the regulatory review process. Natural Cycles has received clearance from the U.S. Food and Drug Administration (FDA) and is CE-marked in Europe. It is also approved for use in Canada, Australia, Singapore, and South Korea.
These regulatory clearances classify Natural Cycles as a medical device, meaning its algorithm, effectiveness, and risk profile have been reviewed by independent bodies. The approvals are based on scientific evidence demonstrating that the app can be an effective method of contraception when used according to its instructions.