The menstrual cycle is a series of events that occurs every 21 to 38 days (approximately) in a woman’s body that prepare it for pregnancy. Most people have no idea how complex the menstrual cycle is. Instead, we think of the menstrual cycle as simply having two parts: when a woman is having her period (or about to have her period if she gets PMS) and when she isn’t having her period. But it’s nowhere near as simple as that. For most women between puberty and menopause, menstrual cycles will often last from 28 to 29 days, although it’s perfectly normal for a cycle to be from 21 to 38 days, and sometimes even longer in teenage girls. During each menstrual cycle, there are subcycles called an ovarian cycle, ovulation, and a uterine cycle, and within those there are phases: a follicular phase, a luteal phase, a menstrual phase when she has her period, a proliferative phase, and a secretory phase. The hormones that create the menstrual cycle are relased by the hypothalamus and pituitary glands, and by her ovaries and uterus. The main hormones that orchestrate the different cycles and phases are Gonadotropin releasing Hormone (GnRH), Follicle Stimulating Hormone (FSH), Leutinizing Hormone (LH), estrogen and progesterone. To give you an idea of how this might impact a woman’s body, her hypothalamus releases GnRH every 60 to 120 minutes, and over the course of each menstural cycle, her levels of estrogen will vary by 200 percent and her levels of progesterone will vary by over 1200 percent. (If men had that many changes going on in our bodies every month, the world would be an even less safe place than it is now.) One thing a lot of us get wrong is we assume the term “menstrual cycle” means when a woman is having her period. Yet periods are only one part of a woman’s menstrual cycle. Periods will often last from 3 to 5 days, with a normal range being from 2 to 8 days, while an entire menstrual cycle is from 21 to 38 days on average. As for when periods occur during each menstrual cycle, it seems like a woman’s period would be at the end of her cycle, but the opposite is true. The first day of her period is counted as the first day of her menstrual cycle. That’s because starting on the first day of a woman’s period, her ovaries begin the process of producing an egg, which ends in ovulation about 14 or 15 days later, assuming her menstual cycles are a perfect 28 days. (Many women’s menstrual cycles are not this regular or predictable.) It’s important to get the counting right if you are trying to determine when she is the most fertile, because she is most fertile during ovulation. To understand more about menstrual cycles, see menstrual flow, endometrium, cervix mucus.