A menstrual cycle that's off schedule doesn't always mean you're pregnant. Here are seven other reasons your period could be late.

If you're waiting for a late period, there's one thought that's likely going through your mind: Am I pregnant? "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

But mensuration could be late for a variety of reasons, and it doesn't always point to pregnancy. This could be disappointing to learn if you're TTC or a huge sigh of relief if you're not ready for a baby. Before you run to the local pharmacy to pick up an at-home pregnancy test, read about these external factors and conditions that could cause delayed periods.

1. Your Birth Control

Birth control often regulates your period, but women who take extended-cycle birth control pills (like Seasonique, Seasonale, or Quartette) won't experience menstruation on a typical 28-day cycle. That's because these contraceptives work on a 91-day cycle—meaning your period will come every three months.

"When you keep taking the active pills for more than the usual 21 days, the lining of the uterus stays stable," says George Patounakis, M.D., Ph.D., FACOG, Progyny's fertility specialist in Florida. "Once you take the inactive pill, hormone levels drop and trigger menses. It's not a period the way you would think of in a woman who's not taking birth control because it's induced by medication and not by normal processes." 

Other types of hormonal birth control—like intrauterine devices (IUDs) and the Depo-Provera shot—can also cause irregular or late periods. However, Dr. Patounakis cautions that no contraceptive is 100 percent effective at preventing pregnancy, so if you don't get your period when expected, you might want to take a pregnancy test.

2. Polycystic Ovary Syndrome (PCOS)

During a typical menstrual cycle, each ovary develops roughly five follicles, and those follicles compete to become the dominant one that releases a mature egg at ovulation. Women with polycystic ovary syndrome (PCOS) often have additional follicles, which makes this process take longer than usual. No released egg means no period.

Other PCOS symptoms include weight gain and increased levels of the testosterone-like hormone androgen, which can cause thick hair growth on the face and breasts. But even without these symptoms, someone can't rule out PCOS. "There are women 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.

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. But it's important to note that each woman experiences stress differently, so its effect on the menstrual cycle is highly subjective, says Dr. McDonald. Moving across the country or dealing with a challenging work project could throw off one woman's period, but the same situation might have no effect on another woman.

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4. Fluctuating Weight

Excessive weight loss is another reason for late periods. "A body mass index (BMI) under 20 creates a starvation-ish mode in the brain," says Dr. McDonald. "That's why some really lean female athletes don't have periods—being underweight creates an environment that's anti-pregnancy." Adds Dr. Vyas: "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."

Gaining body weight can have a similar effect, especially when related to conditions like PCOS. Women with PCOS may be extra sensitive to the numbers on the scale. "As little as 10 percent 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 woman experiences menopause at 51, but before that, they go through a transitional period known as perimenopause. During this time, which usually starts in the 40s, some people have delayed menstrual cycles. Instead of the standard 28 days between periods, for example, 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.

6. A Pituitary Tumor

Though it's rare and unlikely, sometimes a prolactinoma—a type of 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 women who are suffering from 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. "Many women will, for various reasons, occasionally not ovulate and this can cause an early or delayed menses," he says. "Depending on her history, it may require further testing to determine the cause."

When to Visit the Doctor

Make an appointment with your doctor if you can't pinpoint the reasons for a late period; they may want to check for various conditions. It's also important to note that vaginal bleeding after a late period may not be the monthly visitor you were expecting. "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."