The menstrual cycle is one of the most information-dense biological phenomena we know of. Over roughly 28 days (the actual range is 21 to 35 days, with substantial individual variation), a cascade of hormonal events orchestrates processes ranging from follicle development to ovulation to uterine preparation. The hormones involved don't just affect reproductive function; they influence mood, cognition, energy, sleep, immune response, cardiovascular function, and metabolism.
This information has always existed in the body. Until recently, we simply lacked the tools to read it continuously.
Why Blood Tests Cannot Track Your Hormones Continuously
The gold standard for hormone measurement has long been serum blood tests. Draw blood, send it to a lab, wait for results. This approach has genuine advantages: it is accurate, well-validated, and capable of measuring a wide panel of hormones simultaneously.
It also has fundamental limitations that make it unsuitable for continuous monitoring.
A blood test captures a single moment. Hormones can fluctuate dramatically over the course of hours. LH, the hormone that triggers ovulation, can surge and fall within 24 to 48 hours. A blood draw taken a day before the surge shows nothing; a draw taken a day after shows only its aftermath. The window of clinical interest may have already closed by the time results return.
Blood tests also require clinical infrastructure. They require a needle, a trained phlebotomist, a laboratory, and days of waiting. They are not something you can do at home, repeatedly, over the course of a month.
What Modern Wearables Can Detect Through the Skin
Contemporary wearables, including smartwatches, fitness trackers, and continuous glucose monitors, have demonstrated that a surprising amount of physiological information is accessible noninvasively through the skin. Heart rate, blood oxygen saturation, skin temperature, galvanic skin response, and interstitial glucose are all measurable without needles or laboratory equipment.
The question we asked when we started building Cyra was: what else is there?
The answer, it turns out, is a great deal. The skin and the tissues just beneath it are metabolically active environments. The fluid that bathes these tissues, called interstitial fluid, contains many of the same molecules that circulate in blood, including hormones. The concentrations are different, the kinetics are different, but the information is there.
The Biosensor Stack: What Cyra Measures and Why
Cyra uses a combination of sensors to capture physiological signals that correlate with hormonal state. We are not measuring hormones directly through the skin, which remains a significant technical challenge, but we are measuring the downstream effects of hormone activity on multiple physiological systems simultaneously.
Heart rate variability changes with estrogen and progesterone levels. Skin temperature follows a predictable pattern across the menstrual cycle, rising after ovulation as progesterone increases basal body temperature. Subtle changes in skin impedance reflect fluid shifts driven by hormonal fluctuations. Sleep architecture changes with cycle phase in ways that are detectable through motion and heart rate data.
Individually, none of these signals is a reliable hormone proxy. Collectively, processed by the right model, they become something more powerful: a continuous record of hormonal state derived from the body's own responses to its hormones.
How Machine Learning Converts Sensor Data Into Hormone Estimates
The sensors generate raw signals. Translating those signals into hormone estimates requires machine learning.
We trained our models on data from participants who wore Cyra prototype devices while also undergoing regular blood hormone measurements. This allowed us to build a dataset of paired observations: what the sensors see, and what the blood tests confirmed. Over time, and across enough participants, patterns emerged.
The model learns that a particular pattern of heart rate variability, combined with a particular skin temperature trajectory and a particular sleep architecture, correlates with a particular hormonal profile. It learns the individual baseline, since everyone's physiology is different, and it learns how to detect deviations from that baseline that signal hormonal events.
The result is not a direct hormone measurement. It is a probabilistic inference about hormonal state, updated continuously as new sensor data arrives, calibrated to the individual's own physiological patterns.
Hormone Tracking Accuracy: What Cyra Can and Cannot Do
We want to be precise about this, because precision matters in health contexts.
Cyra can estimate your menstrual cycle phase with high accuracy. It can identify the approach of ovulation days before it occurs, giving you advance notice that allows for actual planning. It can detect the post-ovulatory rise in progesterone that confirms ovulation occurred. It can flag patterns that may warrant clinical attention.
Cyra cannot replace blood hormone testing for clinical diagnostic purposes. If you need to know your exact estradiol level on cycle day 3, you need a blood test. Our estimates are calibrated to be accurate at the population level and useful at the individual level, but they are not laboratory measurements.
What we can offer is something blood tests cannot: continuity. Not a snapshot, but a film. Not a single data point, but a pattern across time that reveals the shape of your individual cycle in ways that episodic testing never could.
Why Continuous Hormone Monitoring Changes Women's Healthcare
The practical applications of continuous hormone monitoring extend well beyond fertility tracking, which is the application that tends to get the most attention.
Women with endometriosis experience pain that correlates with hormonal fluctuations. Continuous monitoring can help them understand their own patterns and anticipate difficult periods. Women in perimenopause experience hormonal volatility that is currently invisible to them except through symptoms. Athletes can align their training loads with the phases of their cycle where their physiology supports different kinds of effort. Women experiencing mood symptoms can see whether those symptoms correlate with hormonal patterns.
The data was always there, written in the body. We built the tools to read it.