From tender breasts to mood swings, PMS is no joke — but there's one symptom that may catch you completely by surprise. "The basal core body temperature can change over the course of the menstrual cycle, with a higher temperature occurring during the luteal phase of the menstrual cycle after ovulation," Shannon Clark, MD, FACOG, a double board-certified ob-gyn and professor in maternal-fetal medicine in Texas, told POPSUGAR. That's right — you can run a slight fever leading up to your period, without ever actually being sick.
Dr. Clark explained that hormone fluctuations during your cycle can cause your basal body temperature to increase anywhere from 0.3 to 1.0 degrees Celsius. Translation? If you typically run a cool 98.6 degrees Fahrenheit, your temperature could go up to 100.4. This isn't a huge spike, but it's enough to give you the "period flu," where you feel clammy and hotter than usual. Rest assured: you likely don't actually have the flu! Your body is just doing its thing and gearing up for the main event.
If instead conception occurs, the core body temperature remains elevated until the corpus luteum — a clump of cells that helps produce progesterone during early pregnancy — is no longer functional, which is typically at the end of the first trimester, Dr. Clark added. However, if an egg isn't fertilized, your temperature will drop back down to normal levels after menstruation.
Again, running a low-grade fever during your period is normal. If it's accompanied by other period symptoms like cramps, consider taking an NSAID like ibuprofen to help ease the discomfort. If the fever persists or you're experiencing other symptoms of an infection, it's best to talk to your doctor.