What Are Irregular Periods?

Menstrual cycles don't always run like clockwork. While some women get their periods right on schedule every 28 days, other women are left guessing. Some might skip a month altogether; others might vary from cycle to cycle.

How Many People Have Irregular Periods?

In a South Korean study published in PloS One in March 2019, over 14 percent of 4,709 adult women ages 19 to 54 reported having irregular menstruation. (1) In another, occupational status was found to have an effect: Service, manual overnight shift and sales workers, and unemployed women had higher risks of developing irregular menstrual cycles. (2)

When Is a Menstrual Period Considered Irregular? 

A normal cycle from the first day of one period to the first day of the next can be anywhere from 21 to 35 days, says Becky Lynn, MD, the director of the Evora Center for Menopause and Sexual Health and an adjunct associate professor of obstetrics and gynecology at Saint Louis University in Missouri: “We all think that 28 days is the normal cycle, but there's actually some room for variability.” To determine whether your menstruation schedule is irregular, count from the last day of your previous period and stop counting on the first day of your next. Repeat this for three months. If the number of days between stopping and starting your period is outside of the 21 to 35 days range, you have an irregular cycle. Menstruation can also be considered irregular if your cycle length varies by more than 20 days from month to month. (3)

Why Do Some Menstrual Periods Become Irregular? 

There are many variables that can affect the length and timing of your menstrual cycle. Some are correctable, but others can signal significant medical issues.

• Polycystic ovary syndrome (PCOS) This metabolic and hormonal disorder occurs when the ovaries or adrenal glands overproduce the “male” hormones (androgens) and the body has insulin resistance. (4) Studies have shown that 87 percent of women with irregular menstrual cycles have PCOS. (5)
• Thyroid or pituitary disorders Hypothyroidism (when your thyroid gland doesn't produce enough thyroid hormone), hyperthyroidism (your gland produces too much), and hyperprolactinemia (you have too much prolactin, a hormone from your pituitary gland, in your blood) can all affect menstrual regularity, says Dr. Lynn. One study found that 44 percent of study participants with irregular periods also had thyroid problems. (6,7,8)
• Pelvic inflammatory disease (PID) An inflammation of the female reproductive system, PID is usually caused by sexually transmitted infections. (9)

Irregular Periods Can Be Caused by Non-Disease-Related Issues

Lynn points out that irregular menstrual cycles can also be caused by:

• Perimenopause As you enter the transition to menopause, which can last as long as 10 years, your menstrual cycles will become erratic as part of the process. If you skip more than two periods, you are probably in late perimenopause. (10) It’s been estimated that 70 percent of women experience menstrual irregularities in the approach to menopause. (11)
• Stress and anxiety Chronic stress or even short-term anxiety about a specific problem can wreak havoc with your hormone balance, causing a missed period and irregular cycle.
• Extreme exercise Exercising too much can throw off the timing of menstrual bleeding and sometimes stop it.
• Eating disorders, extreme dieting, or weight loss Being underweight, whether from extreme exercise, dieting, an eating disorder, or illness, can have the same effect.
• Age As mentioned, perimenopause and menopause have a major effect. But being young can create irregularities as well. “Once menstruation begins in young women, it can take several years before it falls into a pattern,” says Lynn.
• Birth control And not just birth control pills; IUDs, implants, and rings can also cause irregular bleeding.

Irregular Periods May Occur While Breast-Feeding a Newborn

Nature’s birth control, exclusive breast-feeding may cause amenorrhea — no menstruation at all — because you are less likely to ovulate for up to about six months. (Learn more before you use this method; many experts recommend new moms pair this approach with an additional birth control method, such as condoms, to increase effectiveness.)

Irregular Periods May Occur After Miscarriage, Pregnancy Loss

Also referred to as a spontaneous abortion, a miscarriage happens when an embryo or fetus dies before the 20th week of pregnancy. After 20 weeks, pregnancy loss is considered a stillbirth. (12) “After these events, it can take a couple of months before you will start having regular periods again,” says Lynn.

Should You Be Concerned if Your Period Is Irregular? 

A couple of irregular periods per year are usually nothing to worry about. Any more than that, and you should see a doctor to be sure an ovulation problem or health condition isn’t the cause. “If you are someone who's not on contraceptives and not of age for perimenopause or menopause and your periods are irregular, you should alert your physician. For example, if you do have PCOS, an irregular cycle can increase your risk of uterine cancer,” says Lynn.

Can a Missed Period Be Considered Part of an Irregular Period?

Sometimes, a period is late for a very good reason. Before you schedule an appointment with your doctor, you might want to rule out pregnancy.

Treatment Options: Can You Fix Irregular Periods? 

If stress is a possible culprit in your irregular cycle, try stress management techniques, such as meditation, yoga, tai chi, visualization, cognitive behavioral therapy, and biofeedback. Avoid over-exercising, and try not to diet excessively. If you need help figuring out what’s best for you, contact a registered dietitian. For all medical issues, consult your healthcare professional.

Can You Get Pregnant With an Irregular Period? 

Absolutely yes, says Lynn. “Sometimes you can ovulate with irregular cycles, sometimes you don’t ovulate with irregular cycles. If you're not ovulating, you're not going to get pregnant, but if you are ovulating with irregular cycles, you can. You can have bleeding without ovulating but it's a real period only if you ovulate. You should use contraception if not desiring pregnancy and to protect against STDs.”

RELATED: Some Period Tracker Apps Are Useful, Help Women Find Fertile Window

There’s No Simple Formula to Show That You Are Ovulating (but You Can Try)

If you are interested in ovulation timing for family planning or any reason, but you have irregular menstrual periods, see your doctor first. If you do have PCOS or another condition, you can get treated as soon as possible.

Can You Calculate Ovulation With an Irregular Period?

Ovulation calculators are not very helpful for women with irregular menstrual cycles. Just as you can’t assume you are not ovulating if you have PCOS, you also can’t assume that you are. That’s another reason to get checked out by your doctor soon, so you can begin treatment before your fertility window closes.

Although none of these fertility awareness methods (FAM) are foolproof, women with regular or mildly irregular periods may be able to help predict ovulation by monitoring themselves for ovulation symptoms such as changes in cervical mucus or changes in basal body temperature and by tracking their cycles.

Learn more about interpreting your body’s ovulation symptoms from a reputable resource, such as Taking Charge of Your Fertility or Planned Parenthood. Products such as apps, testing kits, and ovulation monitors may help some women figure out when they are ovulating; talk to your doctor before investing time and money in these products.