Feeling cold, shivering, or having “period flu” symptoms is a common part of the menstrual cycle. These chills often appear just before or during the initial days of bleeding, even when the ambient temperature is comfortable. Understanding this symptom requires looking at the physiological changes that occur as the body prepares for and begins menstruation. This cyclical feeling of being cold or feverish results from two distinct biological mechanisms: hormonal shifts and inflammatory compounds.
Hormonal Shifts and Body Temperature
The body’s internal thermostat, regulated by the hypothalamus, is directly influenced by fluctuating reproductive hormones. During the second half of the cycle, known as the luteal phase, progesterone dominates. Progesterone acts as a thermogenic agent, slightly raising the body’s basal body temperature (BBT).
This elevation typically ranges from \(0.3^{\circ}\text{C}\) to \(0.7^{\circ}\text{C}\) compared to the first half of the cycle. Progesterone maintains this higher temperature for 10 to 14 days, setting the body’s thermostat to a warmer level. If conception does not occur, the corpus luteum dissolves, causing progesterone levels to plummet rapidly just before menstruation begins.
This sudden decline in progesterone removes the hormone’s warming effect. As the body attempts to quickly reset its temperature back to the lower baseline of the follicular phase, the disparity between the internal temperature set point and the actual body temperature can trigger a cold sensation or shiver. This physiological readjustment is a primary reason for feeling chilly at the start of a period.
Prostaglandins The Inflammatory Link
A second mechanism contributing to period-related chills involves chemical messengers called prostaglandins. These compounds are produced by the cells of the uterine lining (endometrium) just before it is shed. Their main purpose is to stimulate muscular contractions of the uterus, which helps detach the lining and expel it as menstrual flow, resulting in cramping pain.
While primarily local to the uterus, some prostaglandins can enter the bloodstream. Once in circulation, they act as inflammatory agents, similar to the body’s response to an infection. Prostaglandins are considered pyrogens, meaning they can travel to the hypothalamus and temporarily raise the body’s temperature set point.
This action mimics the immune response, causing a mild, low-grade fever in some individuals. The body reacts to this artificially elevated set point by activating mechanisms to warm itself up, resulting in shivering and chills. This inflammatory pathway is responsible for the systemic, flu-like symptoms, such as body aches and fatigue, that many people experience alongside their period.
When Chills Require Medical Attention
While most period-related chills are benign, persistent or severe chills can signal an underlying health issue. One common concern is iron-deficiency anemia, frequently linked to heavy menstrual bleeding. Significant monthly blood loss depletes the body’s iron stores, which are necessary to produce red blood cells.
Red blood cells carry oxygen, and a deficiency can lead to generalized fatigue, weakness, and decreased ability to maintain body heat, resulting in cold intolerance and chills. If you consistently experience extreme fatigue, cold hands and feet, or lightheadedness, a medical evaluation for iron levels may be beneficial.
It is important to distinguish mild, cyclical chills from a sudden onset of severe chills accompanied by a high fever (\(102^{\circ}\text{F}\) or higher). These symptoms, especially when combined with a rapid heartbeat, confusion, or a sunburn-like rash, are red flags for a serious infection like Toxic Shock Syndrome (TSS). Though rare, TSS requires immediate medical attention, and emergency care should be sought if these symptoms develop while using a menstrual product. For typical period chills, comfort measures like a heating pad, a warm bath, or non-steroidal anti-inflammatory drugs (NSAIDs) can help manage the symptoms.