There’s a special frisson to period changes in your mid-forties. Every deviation from your usual pattern can feel like a harbinger of the menopause transition, also known as perimenopause.

One might spend years staring at their underwear, wondering: am I or aren’t I?

Unfortunately, early perimenopause is characterized by uncertainty. That’s because the transition happens in fits and starts, not all at once.

In comparison, menopause itself is just a moment – specifically, the moment 12 months after a person’s last period. It’s only diagnosed retrospectively, long after their ovaries have stopped making estrogen, which drives the menstrual cycle.

It can be really helpful to know whether you’ve started perimenopause. If your symptoms are derailing work or relationships, a diagnosis opens the door to getting treatment. Here’s what you need to know.

What does perimenopause look like?

On average, perimenopause starts around age 47. A changing menstrual cycle usually heralds its arrival, “and the biggest change women will notice is they skip a cycle”, says Nanette Santoro, a professor of obstetrics and gynecology and menopause treatment researcher at the University of Colorado Anschutz School of Medicine.

The first symptoms start an average of four years before the last period, but it’s different for each individual; your periods may start changing months before menopause, or a decade before.

During early perimenopause, periods aren’t as regular, and are often late or early by at least a week. Period flow might be lighter or heavier, and about 40% of women have hot flashes.

Later in perimenopause, periods are even less frequent – sometimes as much as two months apart. Hot flashes peak at this stage, showing up in about 80% of women. Additional symptoms often enter the picture, including depression, sleep disturbances, and genital and bladder symptoms like vaginal dryness and vulvar burning or itching.

Waning estrogen levels are the root cause of these symptoms. In midlife, after decades of releasing estrogen into the bloodstream on a more-or-less regular schedule, the ovaries start to slow down. That translates to less estrogen in the body at less predictable intervals.

Less estrogen means less period flow, because it plays a major role in regular menstruation by growing the uterine lining that gets sloughed off during periods. Estrogen also helps regulate the parts of the nervous system that control temperature and sleep, so dwindling levels lead to hot flashes and sleep problems. It also controls the cells that maintain elasticity and blood flow in the genitals, leading to reduced resilience, lubrication and sensation.

If your periods have stopped due to removal or medical alteration of the uterus, chemotherapy, use of a hormonal IUD or other progesterone-only contraceptives, or other conditions that disrupt regular cycles, you won’t detect flow changes. But other changes will be noticeable. (A blood test can be useful in these situations; more on that below.)

How do I know if I’m in perimenopause?

If your symptoms are pretty typical and you’re over 45, that’s usually enough data for a clinician to make a diagnosis.

“If you’re 49 and you’re telling me you’ve got hot flashes or you’re having these certain changes, I don’t need a blood test to tell you that this is perimenopause,” Santoro says.

Estrogen tests exist, but an abnormal result isn’t that informative, says Lauren Streicher, a professor of obstetrics and gynecology, and menopause researcher, at Northwestern University’s Feinberg School of Medicine. The tests are accurate, but estrogen levels fluctuate wildly during perimenopause. If the level is normal, you might just be catching an upswing or the beginning of a crash – and a low level could also be normal at various times in a person’s life.

Blood tests can be helpful if your symptoms aren’t typical, or if you’re experiencing them earlier than expected. Thyroid problems and diabetes can also begin at midlife and can lead to cycle irregularity, hot flashes and mood swings, so it’s worth ascertaining the true cause.

For particularly confusing scenarios – for example, if you have an IUD and ambiguous symptoms – the blood test for anti-Mullerian hormone (which tells you approximately how many eggs you have left) can help determine whether you are near menopause, but it isn’t useful for every age group and situation.

Several companies sell direct-to-consumer test kits that enable people to check hormone levels at home, but both Streicher and Santoro say they are not good enough predictors of menopause to be particularly useful.

I think I’m in perimenopause – do I need to do anything?

If you have any of the core menopause symptoms, see a menopause specialist about treatment. Hormonal and non-hormonal therapies are available for managing most menopause symptoms, but they’re underprescribed. An OB-GYN with menopause expertise or a primary care clinician with training in treating menopause symptoms can help determine what could work best for you.

While not directly related to menopause, lifestyle basics become particularly important in midlife; Santoro recommends getting good sleep, quitting smoking, exercising regularly and eating healthy, high-fiber foods.

Additionally, get checked regularly for common midlife medical conditions, like high cholesterol, high blood pressure and diabetes. “If you’ve got hypertension in your family and you inherited those genes, they’re coming after you at this time of life,” says Santoro.

• Keren Landman MD is an independent health reporter who is also trained as an infectious disease physician and epidemiologist, with experience serving as a disease detective at the CDC and conducting HIV and malaria research in resource-poor countries. Her public health newsletter is called Landmansplained