Many people experience feeling cold, shivering, or having “fever-like” chills just before or during their menstrual cycle. This reaction is often referred to informally as “period flu,” but it is not a true viral illness and is not contagious. Instead, these chills are a common physiological reaction resulting from biochemical shifts and hormonal changes that occur during menstruation. Understanding the mechanisms behind these chills can help explain why they feel so similar to being sick.
The Inflammatory Response: Prostaglandins and Fever Mimicry
The primary scientific explanation for menstrual chills lies in the body’s inflammatory response, specifically involving hormone-like lipids called prostaglandins. These compounds are produced by the cells of the uterine lining (endometrium) just before and during the beginning of the period. Their function is to initiate the strong uterine contractions necessary to shed the lining, which results in menstrual cramping.
When the body produces an excess of prostaglandins, they can enter the bloodstream and circulate throughout the body. These molecules travel to the hypothalamus, the area of the brain responsible for regulating the body’s temperature set point. Prostaglandins act like pyrogens, substances released during an actual infection that trigger a fever response.
By mimicking pyrogens, the excess prostaglandins trick the hypothalamus into raising the body’s internal temperature set point. When the body senses that its current temperature is below this new, higher setting, it initiates mechanisms to generate heat. This process includes shivering and peripheral vasoconstriction, which are experienced as chills. The body is essentially shivering to warm itself up to a temperature that the brain has determined is the new normal.
The concentration of these prostaglandins is highest during the first 48 hours of menstruation. This is why the chills and other flu-like symptoms, such as body aches and nausea, tend to peak during this initial phase. This mechanism of action directly links the uterine process of shedding its lining to a systemic response that mimics the feeling of having a fever or the flu.
How Hormonal Fluctuations Affect Thermoregulation
A separate cause of feeling chilled is the dramatic shift in reproductive hormones, particularly the sudden drop in progesterone. During the second half of the menstrual cycle, known as the luteal phase, progesterone is secreted in high amounts. Progesterone is naturally thermogenic, meaning it is a heat-producing hormone that causes an elevation in core body temperature.
This elevation typically ranges from 0.3°C to 0.7°C above the baseline temperature, maintaining a higher thermal set point in the body throughout the two weeks leading up to the period. If pregnancy does not occur, the corpus luteum, which produces progesterone, begins to degenerate. This causes a rapid decline in progesterone levels just before the onset of menstruation.
As progesterone levels crash, the body’s core temperature set point quickly falls back to its lower, pre-ovulation baseline. This sudden and significant shift from a prolonged higher temperature to a lower one can make the body feel acutely cold. The resulting chills are a direct consequence of the body adjusting to this rapid thermal decrease, independent of the inflammatory effects of prostaglandins.
Strategies for Relief and Comfort
Since prostaglandins are a major cause of the chills, an effective strategy for relief involves using non-steroidal anti-inflammatory drugs (NSAIDs). Medications like ibuprofen or naproxen work by inhibiting cyclooxygenase (COX) enzymes, which are necessary for prostaglandin production. By reducing the synthesis of these inflammatory compounds, NSAIDs can lessen both the intensity of cramping and systemic symptoms like chills and body aches.
For the best effect, NSAIDs should be taken at the first sign of menstrual discomfort or just before the period is expected to begin. Beyond medication, simple comfort measures can help counteract the feeling of coldness. Applying external heat, such as using a heating pad on the abdomen or taking a warm bath, can help increase overall body warmth. Maintaining hydration by drinking plenty of fluids is also a helpful measure to support circulation and comfort.
Warning Signs: When to Consult a Doctor
While mild, transient chills are a normal part of the menstrual cycle, persistent or severe symptoms warrant a medical consultation. It is important to distinguish between typical menstrual chills and a true fever, which suggests an underlying infection. Seek medical attention if the chills are accompanied by a fever, generally defined as a temperature above 100.4°F (38°C).
Other red flags include chills paired with unusually severe or sudden pelvic pain, or symptoms that continue long after the first few days of the period. These signs could point to a more serious condition, such as pelvic inflammatory disease (PID), a severe uterine infection, anemia, or a thyroid disorder. A doctor can perform testing to rule out these possibilities and offer appropriate treatment.