For many women, the menstrual cycle lasts between 25 and 31 days. Throughout the cycle, hormones fluctuate via communication between the brain, ovaries, and uterus. The main hormones that play a role in this process are estrogen (estradiol), progesterone, luteinizing hormone, and follicle-stimulating hormone. Since the concentration of all of these hormones varies throughout the menstrual cycle, we talk about two main phases: the follicular and the luteal, separated by ovulation.
In general, we have greater hormone levels during the luteal phase—with progesterone higher than estrogen—as the body prepares to nourish a fertilized egg. Hormone levels drop if there’s no fertilization, leading to the follicular phase, but estrogen tends to be higher than progesterone.
That differing ratio of hormones at each phase seems to influence our glucose control, as the body tries to conserve glucose for the parts of the process that are more energy-demanding, like ovulation and building up the uterine lining.
Research shows that glucose tends to be higher during the luteal phase post-ovulation. The higher levels of progesterone (compared to estrogen) reduce insulin sensitivity, which means insulin isn’t as efficient at clearing glucose from your system, leading to higher circulating glucose.
By contrast, glucose levels tend to be lower during the follicular phase, when estrogen is the dominant hormone. Estrogen has several beneficial metabolic effects, including modulating body fat and improving insulin sensitivity. (Indeed, when estrogen levels drop postmenopause, we sometimes see increased insulin resistance and risk of developing type 2 diabetes.)
It’s worth noting that oral contraceptives work by interfering with these normal hormone level changes and thus may affect the glucose effects. Still, research is mixed, and factors such as type (combined OCP vs. the mini pill), dose, potency, and androgenicity will influence a person’s hormonal profile.