Two weeks before your period
Your brain's pituitary gland unloads two fast-acting agents, follicle-stimulating hormone and luteinizing hormone, into your bloodstream. Both direct-message your ovaries: Drop the package!
Message received. Your ovaries jettison their most mature egg into a fallopian tube. This isn't NASCAR, though; that egg's journey to the uterus will take a few days.
In the meantime, your ovaries ramp up estrogen production. The hormone signals the lining of your uterus to start building a suitable home for an embryo.
In the next two weeks
As the uterus builds up tissue and blood supply, your ovaries unleash the key pregnancy-supporting hormone progesterone. Your body temperature may rise a few degrees, though you might not even notice. Other side effects, however, aren't so benign. . .
Progesterone also fuels the expansion of the milk ducts in your breasts. Chances are, your boobs are now swollen and achy. (Women on the Pill may be spared this effect.)
Progesterone may also interfere with certain brain chemicals, including the mood-regulating hormone serotonin. And it can stimulate the amygdala, a brain structure tied to emotion. The result is two charming PMS symptoms: irritability and anxiety.
Meanwhile, estrogen and progesterone are also hard at work preparing the womb. Your intestines may relax a bit to make room for a soon-to-be-occupied uterus. As they expand, so do you (yup, we're talking bloating and gas). Changes in your insulin sensitivity could also trigger food cravings.
As your period starts
That unfertilized egg has waited around long enough, and your uterus senses all the fuss was for nothing. Your estrogen and progesterone levels plummet, along withÃ¢â‚¬”hurray!Ã¢â‚¬”most PMS symptoms.
Yet the fun's not quite over. Your uterine cells begin releasing chemicals called prostaglandins that help slough off the extra blood and tissue. They force your uterine muscles to contractÃ¢â‚¬”a process otherwise known as cramps.
Inflammatory in nature, prostaglandins can also cause nausea. (Their production can often be curbed by exercise or anti-inflammatory meds such as ibuprofen.)
Though you may feel like you're gushing for days, the average blood loss during a period is somewhere between a few tablespoons and a cup.
As your period ends
You made it! But ever the optimists, your ovaries start slowly prepping the next egg for releaseÃ¢â‚¬”so the process can begin all over again.
Sources: Serdar Bulun, M.D., Northwestern University; Robert Casper, M.D., University of Toronto; Nicholas Panay, MRCOG, Chelsea & Westminster Hospital, London