A typical menstrual cycle lasts between 24 and 38 days, with an average of 28 days. A normal period lasts between two and seven days, with an average of five days. If you’re overweight or underweight, your periods are more likely to be irregular.

What Is a Healthy Weight?

Rather than looking at just the number on your scale, it’s important to know your body mass index (BMI). BMI is a way to gauge how much body fat you have. To determine your BMI:

Weigh yourself Measure your height in inches, then square it (multiply it by itself) Divide your weight by your height squared Multiply by 703

So if you weigh 150 and you’re 65 inches tall, the formula would look like [150/(65x65)] x 703 = 24.96. That number then determines whether your weight is considered normal/healthy as opposed to underweight, overweight, or obese.

How Weight Affects Your Period

Being underweight or overweight can alter your menstrual cycle. The menstrual cycle is a result of a complex interaction between your ovaries and your brain.

Changes in certain hormone levels cause ovulation, and more hormonal changes result in your period. Anything that interferes with this interaction can stop your body from ovulating. If you don’t ovulate, you’ll skip a period. 

Hormone levels are affected by your weight and the amount of fat on your body. If you are underweight, with too little body fat, you may skip periods. Too much fat can also lead to missed periods or heavy periods. Rapid weight fluctuations can also interfere with your menstrual cycle.

If you are underweight or very overweight and not menstruating, achieving a healthy weight will likely restart your regular period. 

Effects of Weight Gain

Gaining weight can alter your menstrual cycle in a few different ways. If you were starting at a normal weight and weight gain pushes you into the overweight or obese categories, you may see a change in your periods. If you are underweight and not getting a period, gaining weight could help regulate your cycles.

Infrequent Periods

Women who were normal weight then gained enough pounds to become overweight can begin to have infrequent periods. Increasing your body’s fat stores (also known as adipose tissue) leads to a hormonal imbalance that can stop ovulation. Adipose tissue produces extra estrogen that can hinder ovulation and cause missed periods.

The excess estrogen associated with obesity can increase your breast and uterine cancer risk. Losing weight will restore your regular periods and correct your estrogen excess.

A common cause of missed periods in overweight women is polycystic ovary syndrome (PCOS). PCOS can hinder ovulation and cause you to miss periods. In addition, it causes the ovaries to produce excess androgens, a male sex hormone that interferes with the menstrual cycle.

The greater your BMI (particularly in the obese range over 35), the more likely you are to miss your period. It is even possible to stop bleeding altogether, a condition known as secondary amenorrhea.

Heavier Periods

Women who are obese are more likely to experience heavy periods and abnormal uterine bleeding. This is likely because systemic inflammation from obesity can delay endometrial repair and increase menstrual blood loss. 

Heavy menstrual bleeding (menorrhagia) is defined as bleeding that lasts longer than seven days or is very heavy—by definition, needing to change your tampon or pad after less than two hours or passing clots the size of a quarter or larger. 

Untreated heavy menstrual bleeding can lead to iron-deficiency anemia, a common blood problem that causes fatigue and weakness. If you experience heavy periods, talk to your gynecologist. 

Normal Cycle

If you are underweight and not getting your period regularly, gaining weight can help to regulate your menstrual cycle.

A low BMI is typically caused by calorie restriction, excessive exercise, or illness. These stress your body and cause hormonal changes that interfere with ovulation. This also causes a very low estrogen level, which is especially bad for your bone health.

When you gain weight from a low BMI, you are reducing the stress on your body. This allows your body to ovulate again, and as a result, menstruate. It also restores your body’s estrogen production and protects your bones.​​

Effects of Losing Weight

For women who are obese or overweight and have irregular cycles or heavy menstrual bleeding, losing weight can help your periods become lighter and regular. However, losing too much weight isn’t good either.

Being underweight can cause you to not have a period. This commonly occurs in competitive athletes and women with eating disorders. Women need at least 22% body fat to menstruate regularly. Having a BMI of 18.5 or under can impact your period.

As with weight gain, there is no defined amount of weight loss that results in missed periods when starting from a normal weight.

Light and Infrequent Periods

The more weight you lose and the faster you lose it, the more likely your period will be affected. If you lose weight from significant calorie restriction and strenuous exercise, it may cause a stress response that alters your hormone levels, causing your periods to be lighter and less frequent.

Amenorrhea

If you lose too much weight, you may stop having periods altogether. When you do not have a period for three months (and are not pregnant), it is known as amenorrhea.

Being underweight causes a change in hormone levels, including a drop in estrogen. This interrupts ovulation and causes you to miss your period. If you do not ovulate, you cannot get pregnant. In addition to causing infertility, lower estrogen levels are harmful to your bone health.

Other symptoms of amenorrhea include:

Acne Excess facial hair Hair loss Headache Pelvic pain Vision changes

Treatment

Having regular periods is a good indicator of relative hormonal balance in your body. Both the extremes of being very underweight or very overweight result in hormonal imbalances that stop your periods and over time can lead to serious health issues.

If you experience abnormal menstrual bleeding, irregular cycles, or have stopped getting your period altogether, talk to your gynecologist.

Self Care

Hormonal imbalances can often be corrected by either gaining or by losing weight to achieve a healthy BMI. Before starting on a plan to gain or lose weight, talk with your healthcare provider, a nutritionist, and maybe a personal trainer. Your goal should be to lose fat, not lean body mass, if you are overweight and to gain lean body mass, not just fat, if you are underweight.

When to See a Healthcare Provider

If you have experienced unexplained weight loss or weight gain, talk to your healthcare provider to see if there are underlying health conditions causing your weight to fluctuate.

You should also schedule a visit if you are unable to lose weight despite your best efforts. You could have a metabolic disorder that can be managed with medications along with diet and exercise. In addition, if you have comorbid health conditions that make exercise difficult, talk to your healthcare provider about medications and physical therapy.

Some people struggle to stick to a diet and exercise plan due to an undiagnosed eating disorder. Women who are underweight may experience anorexia or selective eating disorder. Women who are overweight may have bulimia, binge eating disorder, or night eating syndrome.

If you think you may have an eating disorder that is affecting your ability to gain or lose weight, seek treatment. For more information, visit the National Alliance for Eating Disorders website.

A Word From Verywell

Make lifestyle changes that will last the rest of your life, and set realistic and achievable goals. Maintaining your body weight within the normal BMI range (18.5 to 24.9) is one of the most important steps in achieving good overall health.

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