From the onset of menstrual cycles until menopause,  every month a woman’s reproductive system revolves around the task of ovulation - releasing an egg ripe for fertilization. The follicular phase of the menstrual cycle begins the day your period starts and lasts for about 12 to 14 days until you ovulate, or release the egg. During the early part of this stage of the cycle, the female sex hormones estrogen and progesterone are at their lowest levels. Another hormone, follicle-stimulating hormone, is produced, which turns on estrogen production.

This causes the ovary to release an egg midway through the cycle. After egg release, the luteal phase takes over. A second pituitary hormone, luteinizing hormone, triggers the ovary to produce estrogen and progesterone. These hormones cause the lining of the uterus to thicken, in preparation for a possible pregnancy. If fertilization does not occur, the ovary stops making estrogen and progesterone.  The sudden loss of estrogen and progesterone cause the shedding of the uterine lining, and menstruation occurs.

Some women find that the high levels of estrogen and progesterone about a week or so before menstruation affect their blood glucose levels. Researchers aren’t exactly sure why, but they have some clues. Insulin works by binding to receptor proteins that sit on the surface of cells. After insulin binds, it sets off a “relay race” within the cell that in the end allows glucose to enter the cell. When levels of progesterone and other progestin hormones are high, insulin action within cells is affected.

This leads to temporary extra insulin resistance - the cells no longer respond to insulin the way they should. The result is that blood glucose levels may be higher than usual and then drop once menstruation begins.

However, higher than normal estrogen levels may actually increase sensitivity to insulin by improving insulin action.

When this occurs, the increased insulin action can lead to blood glucose levels that may be lower than usual before menstruation.

Not all women experience changes in blood glucose levels before menstruation. Some studies have shown no differences in blood glucose levels throughout the menstrual cycle. Some women experience bloating, water retention, weight gain, irritability, depression, and food cravings, especially for carbohydrates and fats. If you have a tendency to crave these foods, they could also be contributing to high blood glucose levels before your period.