The absence of a menstrual cycle can seem like a dream come true, especially when someone is trying to get pregnant. In other cases, the lack of a period — also known as amenorrhea — can be a cause for alarm.

Missing or not having a period can worry many women because it is seen as a rite of passage and a signifier of adulthood; when menstruation is lacking it can cause a lot of stress.

“There is something magical when your body is in sync hormonally,” Dr. Sherry Ross, an OB/GYN at Providence Saint John’s Health Center in Santa Monica, Calif., told Medical Daily. “When your periods are monthly, a lot of women feel more emotionally and physically balanced.”

Under normal circumstances, menstruation occurs every 28 days, and for some women, the cycle may be more or less frequent. It continues until women reach menopause, which occurs when a woman is 51, on average. Before then, there might be a number of reasons for a woman to miss her period — the factors can range from the physiological to the environmental.

Amenorrhea happens to approximately 2 to 5 percent of women in the United States, and in female athletes it can be much more common. Sometimes a missed period will get back on track within a month. But when it doesn’t resolve itself, that can signal a deeper medical condition.

Ross advises women to see a healthcare provider after one or two irregular periods. “It is definitely something to pay attention to,” she said.

Polycystic Ovary Syndrome (PCOS) affects 5 to 10 percent of women, according to Ross. It’s caused by multiple cysts on the ovaries, and even though they are not harmful, they do cause hormonal imbalances in the body — leading to irregular or missed periods. With PCOS, “your hormones — estrogen and testosterone — are completely lopsided and irregular,” Ross said. “Your periods can come every two weeks, every 3 to 6 months, or once a year.” She advises women to seek medical attention if their periods come more often than every 21 days or less frequently than every 45 days.

In some instances, certain pharmaceuticals like birth control pills, aspirin, and ibuprofen can change the hormones in the body that regulate the menstrual cycle, Ross said.

Body fat plays an important role in menstrual regulation. “Having too little body fat, in the case with anorexia, can cause periods to stop altogether,” Angela Grassi, a registered dietician from Bryn Mawr, Pa., told Medical Daily. She said that women whose body mass index is under 20 will tend to see a decrease in menstrual regularity.

According to Summit Medical Group, athletic amenorrhea, which is caused by overexercising, is common in sports that put pressure on the athletes to be thin, such as gymnastics and long-distance running. This is because drastic weight loss affects the body’s leptin production.

Also known as the fat hormone, leptin regulates fat storage in the body. “Excessive exercising and drastic weight changes can decrease body fat, causing this and other hormones to drop, contributing to irregular periods,” Ross said.

Stress can be both a physical and mental detriment to many bodily functions, including menstruation. “That is because when someone is stressed, the body sends signals to the brain telling it to stop releasing estrogen and progesterone, both needed for menstruation to occur,” said Grassi.

Thyroid hormones, like thyroxine and levothyroxine, regulate every function in the body. When these don’t work properly, a missed period can occur. “Thyroid dysfunction is one of the most common reasons for irregular periods,” Ross said.

Other hormonal causes can include poor control of diabetes mellitus, premature ovarian failure, and Cushing’s disease — a condition that is caused by high cortisol.

Fortunately, healthcare providers can manage these issue, Ross said. Using the right balance of diet, exercise, and hormones can help to regulate a menstrual cycle.