If you've ever had a period, you probably know that they aren't always on time. Cycles usually last between 21 and 35 days, and it's normal for them to fluctuate a few days here and there. But sometimes there are months where you just skip it entirely, and when you're supposed to be menstruating but you've gone three months without a periods, that's called amenorrhea. 

Sometimes, of course, skipping periods means you're pregnant, and it can also happen breastfeeding or using certain kinds of birth control. Other times, it's because of medical conditions that directly interfere with hormones. But there are other things that can make you miss a period too, because having enough energy is really important for keeping your body working properly. 

Here's how the menstrual cycle works, normally: It's kicked off by the hypothalamus, a gland in the brain that releases a hormone called gonadotropin releasing hormone. This hormone causes the pituitary gland to release two more hormones, which regulate the ovaries and the production of the two hormones that affect menstruation most directly; estrogen and progesterone. 

Among other h=things, estrogen helps build the uterus lining that would nourish a fertilized egg. If the egg isn't fertilized, it breaks apart, and the lining and the egg are shed during menstruation. But this process can be disrupted if there isn't enough energy available, like if you're exercising too much or too hard. Which would explain why amenorrhea is most common among ballet dancers, gymnasts, and long distance runners. 

One study in Penn State University found that about half of exercising women experience at least some kind of change in their menstrual cycle. And, a calorie deficiency can stop periods, too. In fact, amenorrhea is one of the diagnostic criteria for anorexia. Exercise and too few calories both affect the menstrual cycle because they have a similar effect on the body, an energy deficiency. 

Energy deficiency puts your body in preservation mode, the body focuses on conserving fuel for the most important functions, and reproduction doesn't make the cut. The amount of energy you use at rest goes down, and you produce hormones at different levels. You produce less gonadotropin releasing hormone, which means the pituitary gland doesn't release the hormones that regulate estrogen and progesterone. And that means no period. 

Lifestyle changes can usually bring it back, basically by making sure you're getting enough nutrients. But there can be dangerous side effects in the meantime, mainly when it comes to bone health. Estrogen helps maintain bone density, and people with amenorrhea tend to have low estrogen levels. Which means that even people who are very young can be at risk of developing osteoporosis if they have amenorrhea. 

So, if you're supposed to be having periods and then you skip one, you should probably get that checked out. 

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