Catamenial epilepsy is a form of epilepsy in women where seizures are exacerbated during certain phases of the menstrual cycle. In rare cases, seizures occur only during certain parts of the cycle; in most cases, seizures occur more frequently (but not exclusively) during certain parts of the cycle. Catamenial epilepsy is underlain by hormonal fluctuations of the menstrual cycle where estrogens promote seizures and progesterone counteracts seizure activity.

Since at least ancient Greek times, there has been documented studies of women with epilepsy and its correlation to the menstrual cycle. So catamenial epilepsy is a unique group of seizure disorders and these seizures are affected mainly by fluctuations in the menstrual cycle of estrogen and progesterone and to clarify the diagnosis of catamenial epilepsy charts of seizure activity are drawn during the menstrual cycle and thus three patterns of catamenial epilepsy are identified.

"Catamenia" was a scientific word for the menstrual period, formed as a neologism in the 18th century, from the Greek katamēnios = monthly (from kata = "by" + mēn = "month").

Levels of the major gonadal hormones estrogen, progesterone, and testosterone vary during the menstrual cycle, and this can trigger catamenial epilepsy. These hormones are synthesized in various locations in the body, including the ovaries, adrenal gland, liver, subcutaneous fat, and brain. There is considerable research showing that these steroidal hormones are important in the pathophysiology of epilepsy. Broadly defined, estrogen and its many forms are thought to be "proconvulsant", whereas progesterone is thought to be "anticonvulsant" by virtue of its conversion to the neurosteroid allopregnanolone.

Estrogen can be found in the female body in various forms, all of which affect women with catamenial epilepsy. Estrone (E1), estradiol (E2), and estriol (E3) are the three principal circulating estrogens in the body. These three forms influence neuronal excitability, but little is known about their inter-hormone interactions, the relative concentrations and ratios of E1/E2/E3 and how that may influence the seizure frequency behavior in women with epilepsy. In normally menstruating women, serum estradiol levels are typically elevated by day 10 of the menstrual cycle, which persists until ovulation.

In general, progesterone administration is anticonvulsant, as supported by experiments in laboratory animals, where injection of progesterone leads to an increase in seizure threshold, or delay to the onset of seizures induced by convulsants.

Similarly to estrogen receptors, progesterone receptors bind several molecules other than progesterone. Progestogens are group of natural non-synthetic hormones, including progesterone, which binds to progesterone receptors. Other than progesterone, progestogens have several neuroactive metabolites, most notably allopregnanolone. Progesterone has been shown to lower the number of estrogen receptors, and thus act as an antagonist to estrogen actions. In trials, both progesterone and allopregnanolone administration have shown a neuroprotective effect on hippocampal neurons in seizure models induced by kainic acid.

The proper classification for catamenial epilepsy has been debatable for several decades. Researchers have defined catamenial epilepsy from the broadest definition of a "greater than" approach indicating an increase in seizure frequency or severity during any specific phase of the menstrual cycle, to a "sixfold increase" in average daily seizure frequency during specific times in the cycle. In recent years, Herzog's 1997 proposal of a twofold increase has generally been accepted: Perimenstrual (C1), Periovulatory (C2), and Luteal (C3). These three classifications are based upon serum estradiol:progesterone ratio, and a 24- to 34-day menstrual cycle in which menses begins on day 1, and ovulation occurs 14 days prior to menstruation. By this measure, approximately one-third of women with epilepsy would be classified under the designation of catamenial epilepsy.