Premenstrual syndrome (PMS) is linked to normal changes in your endocrine system that start when you ovulate, lasting up to the first days of your menstrual period. Contrary to popular belief, ovulation and PMS symptoms don't necessarily start 2 weeks before your period. The day of ovulation varies widely from woman to woman, ranging in one study from day 10 to day 22 of a monthly menstrual cycle.⁸ Women with irregular cycles have an even greater range of possible ovulation and premenstrual days.

Any number of hormone changes can cause premenstrual symptoms-this accounts for the many types of symptoms that women have after ovulation. As your hypothalamus, pituitary gland, thyroid gland, adrenal glands, and ovaries work together to produce an egg (ovum) and prepare your body for a possible pregnancy, they send out chemical signals to each another and the rest of your body. These signals-in the form of hormones and brain chemicals, or neurotransmitters-can affect your mood, energy level, ability to think clearly, body fluid and weight, and pain perception. If one part of the endocrine system isn't working right, the rest of the system is affected, often causing a combination of premenstrual symptoms.

Although most women first experience PMS in their mid-20s, PMS becomes even more common among women in their 30s.⁶ Among women in their late 30s and early 40s (who have erratic periods, ovulation, and hormone changes), unpredictable physical, emotional, and mood-related perimenopausal symptoms can be similar to PMS and premenstrual dysphoric disorder (PMDD).⁵ After menopause, when hormones are low and no longer fluctuating, women do not have PMS.

If you take estrogen and/or progestin for birth control or for hormone replacement after menopause, you can also have PMS-like symptoms.