Menopause starts with a gradual decline in the number of egg-containing follicles in your ovaries, a process that accelerates in the final decade of reproductive life. Most women experience menopause between ages 45 and 55, but the transition leading up to it, called perimenopause, typically begins 5 to 10 years before your last period. That means changes can start in your late 30s or early 40s, often before you realize what’s happening.
What Happens Inside Your Ovaries
You’re born with a fixed number of follicles in your ovaries, each containing an immature egg. Over your lifetime, these follicles are steadily used up through ovulation and natural cell death. By the time you’re still menstruating regularly in your 40s, you have roughly 1,400 primordial follicles remaining. By perimenopause, that number drops to around 140. After menopause, follicles are virtually absent.
What’s notable is that this depletion doesn’t happen at a steady pace. The loss accelerates dramatically in the last decade before menopause, as though the ovaries hit a tipping point. This accelerated decline is what triggers the cascade of hormonal changes that produce symptoms. The shrinking follicle reserve is the immediate cause of both perimenopause and menopause itself.
How Your Hormones Shift
As the follicle count drops, your ovaries produce less estrogen and progesterone. In response, your brain tries to compensate by ramping up production of follicle-stimulating hormone (FSH), the signal that tells your ovaries to mature an egg. Early in perimenopause, FSH levels begin to rise and fluctuate significantly, and luteinizing hormone follows. This creates a period of hormonal instability rather than a clean, linear decline. Your estrogen levels may spike unpredictably one month and plummet the next, which is why early perimenopause can feel so confusing.
This hormonal turbulence is responsible for most of the symptoms women experience. It also has metabolic consequences. The rate at which FSH rises during early perimenopause, rather than your baseline hormone levels, is linked to changes in how your body processes blood sugar and your risk of developing diabetes later on.
The First Signs You’ll Notice
The earliest and most reliable sign that perimenopause has started is a change in your menstrual cycle. As ovulation becomes less predictable, your periods may come closer together or further apart, your flow may shift from light to heavy, and you may skip cycles entirely. A useful rule of thumb: if the length of your menstrual cycle consistently varies by seven days or more from what’s normal for you, you’re likely in early perimenopause. If you go 60 days or more between periods, you’ve probably entered late perimenopause.
Beyond cycle changes, many women notice symptoms they don’t immediately connect to menopause. Sleep disruptions, mood shifts, increased anxiety, difficulty concentrating, and joint stiffness can all appear during this phase. Hot flashes and night sweats may begin well before your periods stop, though they tend to intensify in late perimenopause and the first few years after menopause.
Why Hot Flashes Happen
Hot flashes are one of the most recognizable symptoms of the menopausal transition, and they have a specific biological explanation. Your brain has a built-in thermostat in a region called the hypothalamus, which maintains a “thermoneutral zone,” a range of core body temperatures it considers normal. When estrogen levels drop, levels of a brain chemical called norepinephrine rise, and this narrows that comfortable temperature range considerably.
In women who experience hot flashes, the thermoneutral zone can shrink to essentially zero degrees, meaning even a tiny increase in core body temperature triggers the brain’s cooling response: blood vessels near the skin dilate, you start sweating, and you feel a sudden wave of heat. In women without hot flashes, the thermoneutral zone remains about 0.4°C wide, giving the body more room to absorb minor temperature fluctuations without triggering an alarm. The combination of estrogen withdrawal and elevated norepinephrine is what makes the thermostat so sensitive.
How Long the Transition Lasts
Perimenopause typically lasts between 2 and 8 years. The wide range reflects genuine biological variation. Some women experience a relatively brief transition with mild symptoms, while others spend the better part of a decade navigating irregular cycles and fluctuating hormones. There’s no way to predict in advance how long your transition will take.
Menopause itself is defined retrospectively. You’ve reached menopause once you’ve gone 12 consecutive months without a period, with no other obvious medical explanation. There’s no blood test that definitively diagnoses it in real time. Menstrual irregularity remains the only objective marker clinicians use to confirm you’re in the transition. FSH levels can be measured, but because they fluctuate so much during perimenopause, a single reading isn’t reliable enough to confirm where you are in the process.
Factors That Affect When It Starts
Genetics plays a major role in determining when you’ll reach menopause. If your mother or sisters went through menopause early, you’re more likely to as well. Researchers have identified several genes involved in ovarian aging, though the picture is complex and no single gene determines the outcome.
Smoking is one of the strongest modifiable risk factors for earlier menopause. Women with over 30 pack-years of smoking history (roughly a pack a day for 30 years, or two packs a day for 15) have about 1.5 times the risk of early menopause compared to nonsmokers. This holds true regardless of genetic predisposition. Research using advanced genetic analysis has confirmed that daily cigarette consumption is a direct causal factor, not just a correlation. Smoking appears to accelerate follicle loss and damage the ovaries through multiple biological pathways.
Other factors associated with earlier menopause include never having been pregnant, a history of certain autoimmune conditions, and prior ovarian surgery. Higher body weight is associated with slightly later menopause, likely because fat tissue produces small amounts of estrogen. Living at high altitude, having a lower socioeconomic status, and exposure to certain environmental toxins have also been linked to earlier onset, though these effects are smaller than genetics and smoking.