Everyone is getting on your nerves and did you just tear up at that insurance commercial? Take a look at the calendar because it’s probably your body preparing for your period.

According to the American College of Obstetrics and Gynecology, 85% of menstruating women experience one or more PMS symptoms on a regular basis. These symptoms are experienced in the luteal phase of the menstrual cycle, after ovulation but before menstruation, and can range from the emotional (like irritability and anxiety) to the physical (acne, cramps, and bloating). Most women experience mild versions of these symptoms and don’t require any treatment. However, between three and five percent of women have a more severe version of PMS called Premenstrual Dysphoric Disorder, or PMDD. While most women are likely very familiar with PMS symptoms, what exactly is the science behind these symptoms?

It turns out, science still has some ongoing debates about PMS, including whether symptoms are real, if it’s a result of cultural expectations of women to be more emotional, or an outlet to push back against cultural expectations for women to not be angry. Except none of those reasons explain your terrible back pain every month.

Hormones
Most scientists do acknowledge some form of PMS and point to hormones as one cause. The hormones estrogen and progesterone are neurosteroids, stimulants that affect our brain and help regulate mood and memory.

All women have estrogen and progesterone, but not all women react the same to the changing levels of these hormones throughout your menstrual cycle. Your friend who never experiences any symptoms before her period may have the exact same amount of estrogen and progesterone in her blood. The difference, as scientists currently understand it, is that women who get PMS may be more sensitive to hormonal changes.

Brain chemicals
The shifting levels of hormones change our brains and nervous systems, affecting behavior, emotions, and thoughts. Estrogen alone can improve mood and well being, but progesterone can have the opposite effect. These hormones disrupt two chemicals in the body that cells need to be able to communicate: serotonin and GABA (gamma-Aminobutyric acid). During the week before your period, when progesterone is high and estrogen is low, there isn’t enough serotonin available for cells. Insufficient amounts of serotonin can lead to depression, fatigue, food cravings, and sleep problems.

GABA regulates stress, anxiety, and alertness, calming the neurons in our brain when they might otherwise be firing. In the brain and ovaries, progesterone shows up as the chemical allopregnanolone or allo which helps GABA regulate mood. If progesterone levels change causing both it and allo to become less available, GABA doesn’t work as well to regulate stress. These hormonal changes are why your gynecologist might recommend hormonal birth control to control (but not eliminate) some of your PMS symptoms.

Beyond the brain
As for symptoms that have nothing to do with the brain, like bloating, cramping, and stomach pain, science has other theories for those. Bloating may be due to a hormone in the kidneys called aldosterone. Progesterone inhibits aldosterone leading to water retention. Pain could be from those brain and mood changes making for increased sensitivity to symptoms. Symptoms can also be due to lifestyle: food, lack of exercise, and stress.

PMS does have some cultural roots. For instance, except for cramps, American women (and those in Australia and Western Europe) are more likely to complain of PMS symptoms. Chinese women report being more sensitive to cold before their periods, but that American women do not generally report that symptom. American women also have the highest levels of progesterone in the world, which could be an intersection of culture and biology to cause PMS.

Questions remain unanswered about PMS and why and how it affects us, but scientists agree that more needs to be understood about the biology and psychology behind our monthly symptoms. In the meantime, pass the chocolate.

Camille Acker is a freelance writer living in Chicago and is a co-founder of The Spinsters Union. She is the proud owner of many, many books. See more at camilleacker.com.