You’ve undoubtedly heard the phrase "biological clock" used in the context of a woman’s fertility: It refers to the window of prime babymaking years before she reaches menopause, when her menstrual cycles end. But the timeline towards menopause isn’t as linear—or clear-cut—as it’s sometimes made out to be. Before menopause, women go through a transitional period known as perimenopause. The body's production of the hormones estrogen and progesterone begins to drop off during this time, causing menstrual cycles to shorten, and, ultimately, cease. The transition can last anywhere from two to five years; once a woman has gone without a period for 12 months, she's considered fully menopausal.

Perimenopause onset can vary from person to person, explains Michelle Warren, MD, medical director at the Center for Menopause, Hormonal Disorders and Women’s Health in New York City. In the United States, the average age of onset is 47, but Dr. Warren has seen it begin in patients as young as their late 30s. Symptoms can also differ from person to person—and even from month to month in individuals—making perimenopause "difficult for both patients and doctors to recognize," she explains. Still, there are a few common signs that can indicate a patient is entering perimenopause, she says. If you're experiencing any of these symptoms, talk to your doctor, who can help you come up with a treatment plan to manage them.

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Inconsistent cycle

Dr. Warren says the first red flag is often changes in a woman’s cycle, since fluctuating levels of estrogen can cause menstrual irregularities. Women entering perimenopause may find themselves experiencing varying lengths of time in between periods or skipping some entirely.

Heavy bleeding

Blame this one on changes in progesterone levels: Because ovulation is becoming more erratic, the endometrium—the uterine layer that sloughs off during your period—grows thicker than it would during a normal cycle, causing heavy bleeding to occur. If this is happening to you, your doctor may prescribe low-dose hormones to help you ride out the changes.

Hot flashes

About 75 percent of perimenopausal women suffer from frustrating hot flashes, which can range from low-key flushing to intense sweating. Experts haven’t determined exactly why hot flashes occur, but the decrease in estrogen is believed to play a role, since it interferes with the body’s ability to keep temperatures steady. Avoiding caffeine, spicy foods, and alcohol may help make hot flashes less severe.

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Mood changes

"In my experience, the most common [symptom of perimenopause] is terrible mood changes such as anger and irritability," says Dr. Warren, explaining that these changes can occur very suddenly. (A drop in estrogen is the likely culprit here, as well.) Women who have a history of postpartum depression may have a higher risk of mood changes during perimenopause, she adds.

Vaginal dryness

Yet another unwelcome side effect of a drop in estrogen. Decreased levels of the hormone can cause vaginal tissue to lose its elasticity and natural lubrication, which can lead to pain during intercourse. For an over-the-counter fix, vaginal lubricants and moisturizers may help ease discomfort.