The levels of your reproductive hormones — estrogen and progesterone — rise and fall unevenly during perimenopause. Your menstrual cycles gradually lengthen, and you begin having menstrual cycles in which you don't ovulate. It's only during cycles when you do ovulate that you can become pregnant.

When perimenopause starts and how long it lasts varies. You'll probably notice signs of impending menopause, such as menstrual irregularities, sometime in your 40s. But some women notice changes as early as their mid-30s.

Perimenopause causes some subtle — and some not-so-subtle — changes in your body. Some things you might experience include:

• Menstrual irregularities. The intervals may be longer or shorter, your flow may be scanty to profuse, and you may skip some periods. As ovulation becomes more erratic, the absence of progesterone may lead to longer and heavier periods.
• Hot flashes and sleep problems. About 75 percent to 85 percent of women experience hot flashes during perimenopause. Intensity, duration and frequency vary. Sleep problems are often due to hot flashes or night sweats.
• Mood changes. Hormonal fluctuations may be responsible for the mood swings, depression and irritability you may experience during perimenopause. But these symptoms may also be caused by or aggravated by other symptoms you're experiencing, such as lack of sleep from nighttime hot flashes.
• Vaginal and bladder problems. When estrogen diminishes, your vaginal tissues may lose lubrication and elasticity, making intercourse uncomfortable or painful. Low estrogen levels may also leave you more vulnerable to urinary infections. Loss of tissue tone may contribute to urinary incontinence.
• Decreasing fertility. As ovulation becomes less regular, your ability to conceive decreases. However, as long as you're having periods, pregnancy remains a possibility. If that's not what you want, use birth control until you've had no periods for 12 months.
• Loss of libido. During perimenopause, sexual desire and arousal may gradually decline. The cause may be hormonal, but it can also result from other stresses in your life or your attitude toward aging.
• Body and skin changes. As your estrogen levels decline, your muscle mass decreases, body fat increases, and your waist and abdomen thicken. Lower estrogen levels also affect collagen in your skin, so it gradually becomes thinner and less elastic.
• Loss of bone. As your estrogen levels decline, you may lose bone more quickly than you replace it, increasing your risk of osteoporosis.
• Changing cholesterol levels. Declining estrogen levels may lead to unfavorable changes in your blood cholesterol levels, including an increase in low-density lipoprotein (LDL) cholesterol — the "bad" cholesterol — which contributes to an increased risk of heart disease.

For most women, the discomforts associated with perimenopause are minimal and manageable.

As you age, your body produces less of the reproductive hormones estrogen and progesterone. These hormonal fluctuations are the root of the changes your body goes through during perimenopause.

Certain factors may predispose you to experiencing perimenopause at an earlier stage in life. These include:

• Family history. If your mother experienced an early menopause — at age 45 or younger — chances are you'll experience menopause at a younger age, too.
• Cigarette smoking. Perimenopause occurs one to two years earlier in women who smoke, compared with women who don't.
• Hysterectomy status. Even if your ovaries were spared, surgery that removes your uterus (hysterectomy) may impact how well your ovaries function — and how much estrogen they produce.
• Poor nutrition. Excessively thin or undernourished women may experience menopause at an earlier age.

Some women seek medical attention for their perimenopause symptoms. But others either tolerate the discomfort or simply don't experience symptoms annoying enough to warrant attention. Because subtle symptoms may come on gradually, you may not realize at first that they're all connected to the same thing — declining and fluctuating levels of estrogen and progesterone.

If you do experience symptoms that you just can't tolerate, such as severe hot flashes, radical mood swings, or loss of sex drive, see your doctor.

Perimenopause is a process — a gradual transition. No one test or sign is enough to determine if you're in the perimenopause phase. Before making a diagnosis of perimenopause, your doctor will take many things into consideration, including your age, menstrual history and what symptoms or body changes you're experiencing. Some doctors may also order tests to check your hormone levels. Increased levels of certain hormones, such as follicle-stimulating hormone and luteinizing hormone, are an indirect measure of estrogen levels and can be helpful in assessing menopausal status.

Irregular periods are a hallmark of perimenopause. Most of the time it's normal and nothing to be concerned about. However, see your doctor if:

• Bleeding is continuous and extremely heavy — you're changing tampons or pads every hour
• Periods regularly occur less than 21 days apart
• Bleeding lasts longer than 10 days
• Bleeding occurs between periods

Heavy bleeding doesn't necessarily mean there's something wrong, but your doctor may want to run some tests to determine the cause.

Oral contraceptives are often the treatment of choice to relieve perimenopausal symptoms — even if you don't need them for birth control. Today's low-dose pills regulate periods and eliminate or reduce hot flashes, vaginal dryness and premenstrual syndrome.

As you transition from perimenopause to menopause, your doctor may test your follicle-stimulating hormone levels after you stop oral contraceptives for a short time. From this test, your doctor can get a fairly good idea of whether you've gone through menopause. At that point, if you're still experiencing bothersome symptoms, you can switch to hormone therapy — such as estrogen or combination estrogen-progestin — to treat those symptoms.

Making lifestyle changes may help ease the discomfort of your symptoms and keep you healthy in the long run.

• Good nutrition. Because your risk of osteoporosis and heart disease increases at this time, a healthy diet is more important than ever. Adopt a low-fat, high-fiber diet that's rich in fruits, vegetables and whole grains. Add calcium-rich foods or take a calcium supplement. Avoid alcohol or caffeine, which can trigger hot flashes.
• Regular exercise. Regular physical activity helps keep your weight down, improves your sleep, strengthens your bones and elevates your mood. Try to exercise for 30 minutes or more on most days of the week.
• Stress reduction. Practiced regularly, stress reduction techniques, such as meditation or yoga, can help you relax and tolerate your symptoms more easily.

By monitoring your menstrual cycle and recording your signs and symptoms for several months, you'll gain a better understanding of the changes occurring during this time. You will also have valuable information to discuss with your doctor.