If period comes early one month and late the next

If you're waiting for a late period, one thought may go through your mind: Am I pregnant? But pregnancy isn't the only reason your period could be late. Common reasons you may miss a period when you aren't pregnant include hormonal birth control, hormone-related health conditions, stress, and perimenopause.

"Everybody thinks they're pregnant when their period is late," says Wendy Goodall McDonald, M.D., aka Dr. Every Woman of Women's Health Consulting in Chicago, and the author of It Smells Just Like Popcorn: The Modern Woman's A to V Guide to Her Vagina and Beyond.

So before you run to the local pharmacy to pick up an at-home pregnancy test, read about seven things that could cause a missed period when you're not pregnant.

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1. Your Birth Control

Hormonal birth control can contribute to late periods. For example, intrauterine devices (IUDs) and the Depo-Provera shot—can cause irregular or "late" periods. Also, if you're on birth control pills, skipping the inactive pills (which result in withdrawal bleeding when taken), you'll miss the time you would normally bleed.

Though people often think that birth control pills "regulate" your period, the bleeding people experience during the week of inactive pills is withdrawal bleeding, not a true period. That's because hormonal birth control methods like the pill effectively override your body's natural hormone cycle.

So, when you keep taking the active pills for more than the usual 21 days, the lining of the uterus stays stable, says Dr. Patounakis. That means, the uterine lining won't shed and you won't experience bleeding.

But, with birth control pills, "once you take the inactive pill, hormone levels drop and trigger menses. It's not a period the way you would think of in someone who's not taking birth control because it's induced by medication and not by normal processes," says George Patounakis, M.D., Ph.D., FACOG, an attending physician at Reproductive Medicine Associates of Florida.

However, Dr. Patounakis cautions that no contraceptive is 100% effective at preventing pregnancy, so if you don't get your period when expected, you might want to take a pregnancy test just in case.

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2. Polycystic Ovary Syndrome (PCOS)

People with polycystic ovary syndrome (PCOS) may experience late periods. That's because people with PCOS often have additional follicles, making the menstrual cycle take longer.

During a typical menstrual cycle, each ovary develops roughly five follicles. Those follicles compete to become the dominant one that releases a mature egg at ovulation. When you have PCOS, your egg may release later. No released egg means no period.

Other common PCOS symptoms include weight gain and increased levels of the androgenic hormone, testosterone, which can cause thick hair growth on the face and breasts. But even without these symptoms, someone can't rule out PCOS.

"There are people who are not overweight and don't have extra hair who have irregular cycles, and an ultrasound will show they have excessive follicles," says Anuja Vyas, M.D., FACOG, with Houston Methodist Obstetrics and Gynecology Associates.

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3. Stress

"Emotional distress can affect the region of the brain that controls the pituitary gland, which regulates the hormones that stimulate our ovaries," explains Dr. Vyas. So, as a result, sometimes stress can cause a late period.

But it's important to note that every person experiences stress differently, so its effect on the menstrual cycle is highly subjective, says Dr. McDonald. For example, moving across the country or dealing with a challenging work project could throw off one person's period, but the same situation might not affect another person.

4. Fluctuating Weight

Losing or gaining weight can be another reason for late periods. "Severe weight loss and anorexia can shut down the hypothalamus's production of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) that regulate the ovaries," says Dr. Vyas.

Gaining weight can have a similar effect, primarily when related to conditions like PCOS. People with PCOS may be extra sensitive to weight gain and loss. "As little as 10% weight loss can get them back into their cycle after experiencing irregularity," explains Dr. McDonald. "And a similar percentage of weight gain can cause a late or missed period."

5. Perimenopause

The average American who menstruates experiences menopause at age 51, but before that, they go through a transitional period known as perimenopause. During this time, which usually starts in your 40s, some people have delayed menstrual cycles. Instead of the average 28 days between periods, menses may arrive 36 or 48 days apart. "If you're under 45 and your period stops completely, it's possible you're going through early menopause or experiencing premature ovarian failure," Dr. Vyas adds.

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6. A Pituitary Tumor

Though it's rare and unlikely, sometimes a prolactinoma—a pituitary tumor that secretes excess amounts of prolactin, the hormone that signals breast milk production—is to blame for a late period.

Dr. Vyas says people experiencing irregular periods, headaches, blurry vision, and discharge from the breasts even though they're not breastfeeding may want to get checked by their doctor for this type of tumor.

7. Diabetes and Thyroid Disease

Jay M. Berman, M.D., FACOG, chief of gynecological services at Detroit Medical Center's Harper Hutzel Hospital and professor of obstetrics and gynecology at Wayne State University, says other issues such as diabetes and thyroid disease may be to blame for a late period.

According to the U.S. Department of Health and Human Services Office on Women's Health, thyroid disease can cause problems with your period because thyroid hormones help control your menstrual cycle. Unbalanced thyroid hormones can cause your period to be irregular.

In addition, some studies have found a link between type 1 diabetes and early menopause. Researchers believe this occurs because this type of diabetes causes premature vascular aging and, in turn, ovarian aging. As you reach early menopause, your periods may be late and irregular.

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When to Visit the Doctor

Many factors can affect the regularity of your periods, so the reason behind an occasional late period may not be apparent, according to Dr. Berman. But if it's frequently happening or you're experiencing other symptoms, speak to a health care provider.

"Many people will, for various reasons, occasionally not ovulate, and this can cause an early or delayed menses," he says. "Depending on [the person's] history, it may require further testing to determine the cause."

Make an appointment with your doctor if you can't pinpoint the reasons for a late period; your health care provider may want to check for specific health conditions. It's also important to note that vaginal bleeding after a late period may not be the monthly visitor you expected.

"Anybody who experiences heavy bleeding and pain after a missed period and/or a positive pregnancy test should go to the doctor," says Dr. McDonald. "All bleeding is not a period, especially in a setting where something is off."

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