Up to 70 percent of women with migraine name hormonal changes related to their menstrual cycle as a major migraine trigger. Research has established a fairly clear link between the hormone estrogen and migraine frequency in female migraineurs. Estrogen effects on the brain are complex but can include changes in the levels of the neurotransmitter serotonin, which is linked to the brain changes that occur in migraine.

When estrogen is rising or is holding steady, most female migraineurs experience fewer menstrual-related migraines. It is not uncommon for the frequency of migraine to decrease in pregnancy when estrogen levels are rising rapidly. Alternately, sudden drops in estrogen levels can precipitate migraine. Women who take birth control pills with high levels of estrogen as a contraceptive or to regulate their cycle are also at higher risk for migraine, especially with those preparations where the pill taken during the last seven days of a twenty-eight day cycle is actually an inactive substance. This withdrawal is the trigger for menstruation but leaves the woman most vulnerable to a migraine attack during that week. Migraines tend to resurface during the postpartum period, as estrogen levels drop, but there is some evidence that breastfeeding will extend the migraine improvement period for many women.

While these are the most common observations, the effects of estrogen on migraine are likely quite complex and often unpredictable. For example, estrogen-based hormone replacement therapy (HRT) used for relief of menopausal symptoms may be a migraine trigger for some women. Of great concern for older women who are smokers and who have experienced migraine with aura, there may be a small increase in the risk of stroke when taking birth control or hormone replacement therapy.

Migraine that is directly related to the menstrual cycle with no other identifiable triggers is called menstrual migraine. Most women who have menstrual-related migraines do not consistently experience aura. Fortunately, women who can link their migraines to a predictable menstrual calendar typically have an advantage in treating and preventing their attacks. Using migraine preventative treatments immediately before, during, and again immediately after menstruation is highly effective in many women.

For women with migraines linked to their menstrual cycle, menopause often brings about a decrease in migraine frequency. However, for women who have surgical menopause (i.e., menopause brought about by surgical removal of both ovaries, or caused by ovarian failure following hysterectomy), migraines may actually worsen.

Part of this likely relates to the abrupt drop in hormone levels immediately following surgery, but over time the increase is likely associated with the higher rate of estrogen replacement therapy (ERT) or hormone replacement therapy (HRT) in women who have undergone surgical menopause. In postmenopausal women with a history of migraine who require ERT or HRT, continuous transdermal (i.e., through the skin) treatment that provides a low and steady amount of estrogen to the body is preferred to keep hormone fluctuations to a minimum. This may be in the form of a skin patch or gel. See Chapter 11 for more information on migraine and hormones.