One of the most common concerns is a late or missed period, especially when you know you’re not pregnant. Periods typically happen every 28 days. Irregular periods are very common. They may be due to health issues, but when should they cause worry?

Karen Carlson, MD, OB-GYN, explains when a period is considered late.

“Normal cycles occur every 28 days, plus or minus a week,” says Dr. Carlson. “The cycle starts on the first day of your period. If you’ve gone over 35 days without your period, that would be considered late. Some people go for a few months at a time without a period, and that would be more concerning.”

Hormone imbalances and health conditions are the most common reasons periods are irregular or late, says Dr. Carlson. Aside from pregnancy and breastfeeding, menopause would be a reason as well.

Some of the most common reasons for late periods include:

1. Medications

“Contraceptive use like birth control pills can cause irregular bleeding,” says Dr. Carlson. “Other medications, as well as chemotherapy and radiation therapy, are also reasons.”

When someone stops taking birth control pills or has an intrauterine device, or IUD, removed, there can be a delay in starting a regular menstrual cycle again.

2. Low or high body weight

“Low or high body weight can impact menstrual cycles,” says Dr. Carlson. “Eating disorders like anorexia and bulimia may stop someone’s period.”

Any person can have an eating disorder, and such undernourishment will affect the hormonal functions of regular menstrual cycles. They may be either paused or stopped altogether. A low body weight means less stored fat and thus disrupted reproductive hormone production.

Even those with low body fat who don’t have an eating disorder can be impacted by an erratic or stopped menstrual cycle.

“Cross-country runners or someone who does rigorous ballet training may have deficient body fat, which can cause people to stop menstruating,” says Dr. Carlson.

3. Stress

We all underestimate the effect stress has on our health. Dr. Carlson says physical and mental stress can make people stop ovulating and thus have irregular menstrual cycles.

Being under stress for days, weeks, or even months is called chronic or long-term stress. Aside from your emotional well-being, the hypothalamus is affected. This part of the brain regulates your period and can become thrown off due to chronic stress.

4. Hormone imbalances

“Hormonal imbalances like polycystic ovarian syndrome, or PCOS, thyroid issues, and pre-mature menopause, someone who stops menstruating before the age of 40, can make periods unpredictable or stopped,” says Dr. Carlson.

PCOS causes the body to produce more of the male hormone testosterone. This leads to excessive unwanted hair growth on the body and can make conception difficult.

If the thyroid is over or underactive, the body’s metabolism and hormone levels are affected, potentially leading to late or missed periods.

There are less common but equally serious reasons for a late period. Dr. Carlson explains that menstrual cycles can also be disrupted or halted due to uterine scarring.

Uterine scarring usually happens after someone has a dilation and curettage, or a D and C,” says Dr. Carlson. “The uterine walls can scar together, preventing a normal buildup and shedding of the uterine lining.”

While this isn’t an exhaustive list of reasons periods can be late, it’s some of the most common. If your periods seem irregular or you’re over 35 days late, call 800.922.0000 to schedule an appointment with one of our OB-GYNs.