Every month like clockwork, many women get a visit from aunt Flo, both a blessing and a curse if you're not trying to get pregnant. The physical and emotional symptoms of PMS, from cramps to mood swings, can be intolerable, but at least we don't have to deal with an unplanned pregnancy. Periods are typically predictable, but being a few days late can make us ask the question: “Am I pregnant?”

Our hormones are affected by a number of different factors and body systems. The hypothalamus and brain, pituitary, ovarian, adrenal and thyroid glands all help regulate menstruation and balance hormones naturally. A regular, pain-free period each month is a good sign our hormones are in balance, and the reproductive system is working properly. Meanwhile, irregular periods, missed periods, or extremely painful and severe PMS symptoms are an indication one or more hormone levels are either lacking or are too high.

Many people fear pregnancy when their period is late, but there are many other explanations, especially after a negative pregnancy test.

Below are 6 common reasons why our regular period may be late.

Stress

We all have the built-in "fight or flight" survival response produced by our adrenal glands, which secrete stress hormones like cortisol and adrenaline. These two hormones are related to our stress responses that help us deal with threats, whether they're immediate ones or perceived.  The adrenal glands also communicate with the hypothalamus telling our body to produce more or less of our hormones.

When we're stressed, cortisol increases, delaying ovulation or stopping it altogether.

"When cortisol is high, it...basically pulls away from all the hormones such as estrogen and progesterone, making you have (an) irregular period," Dr. Brandy McGill, a licensed naturopathic doctor in Calif., told Medical Daily.

Thyroid

The thyroid is not perceived to be a likely culprit related to hormonal imbalances. However, the thyroid is part of the endocrine system, which is closely linked to the reproductive system. Hypothyroidism, an underactive thyroid, is accompanied by symptoms including fatigue, weight gain, constipation, hair loss, and heavy periods. Meanwhile, hyperthyroidism, an overactive thyroid, can lead to weight loss, fast heart beat, sleeplessness, restlessness, and lighter periods.

"These patients can also have an altered ovulation and need to try and achieve a normal TSH (Thyroid-stimulating hormone) level prior to attempting pregnancy," Dr. Jane Frederick, a board certified fertility specialist and OB/GYN in Orange County, Calif., told Medical Daily.

Losing Weight

Women who tend to have a body mass index (BMI) that falls below 18 or 19 have a higher probability of missing their period because of too little body fat. Estrogen is stored in our fat, which forms a women's shape, such as her hips and breasts, according to McGill. This is why women normally lose weight in the hips and waist first, because that is where they store the most fat, which can actually help balance our hormones.

Losing too much weight can have a few effects on our hormones.

"If you lose too much weight this can cause stress on the body causing the body to produce the cortisol as mentioned. It can also cause an imbalance in hormones and cause them to be lower," said McGill.

Woman exercising Six reasons why your period is late, other than pregnancy, from thyroid issues to extreme exercise. Photo courtesy of Dominik Wycisło

Poor Diet

A diet low in nutrients, antioxidants, and probiotic foods, but high in stimulants, can take a toll on the adrenal glands and our thyroid. Diets low in fiber and high in saturated fat can cause irregular periods, according to the Women's Health Resource. A healthy diet that consists of plenty of fresh fruits and vegetables is important in promoting normal menstrual cycles.

Extreme Exercise

There is too much of a good thing when it comes to exercise and our period. Exercising at high intensities, like training for a marathon, can stop some women from getting their period. Extreme exercise leading to delayed periods can often be associated with what’s called the “Female Athlete Triad.” This is classified by three things: disordered eating, amenorrhea absence of a period), and osteoporosis, according to Ali Mileski, a certified obstetrics registered nurse in New York City.

"Extreme exercise in the form of aerobic exercise coupled (with) strength training can cause a delay in your period,” Mileski told Medical Daily.

Mileski can attest to this from her own personal experience: “[M]y period was about 10 days late after running my first half marathon.”

A 2012 study in Sports Health found running and ballet dancing are among the physical activities closely linked with amenorrhea. Up to 66 percent of women experience amenorrhea at one time or another.

Dr. Raquel Dardik, a gynecologist at NYU Langone Medical Center in New York City, notes:

“Extreme exercise causing loss of 10 percent of body weight tends to shut down the hypothalamic pituitary axis and cause loss of periods.”

Meanwhile, Dardik also told Medical Daily extreme exercise without weight loss can also lead to missed periods, which is more commonly linked with a cortisol/stress response to the increased exercise.

Contraceptives

Typically, the body starts a period 12 to 14 days after ovulation. If you inhibit or delay ovulation, then you will also extend the time when your period may start. Birth control is highly effective in preventing pregnancy, and promoting regular periods. However, women can still be on the pill, not get their period, and not be pregnant. Hormones in birth control are not the same as what our bodies produce, so they have different reactions to the body.

"Combined oral contraceptive pills, especially the newer low dose ones avoid the rise in progesterone that occurs with ovulation when a woman is not on the pill,” said Dardik.

This makes the uterine lining very thin, which is what sheds monthly. Therefore, the thinner the uterine lining, the lighter the periods.

Emergency Contraception

Emergency contraception is a high dose of of progesterone (0.75mg of levonorgestrel) and it works by preventing or delaying ovulation and tubal transport. It is very similar to the progesterone a woman makes, which can impact the menstrual cycle, possibly delaying menstruation up to a week.