Perimenopause: what it is, when it starts and how long it lasts

Many women notice that their periods start behaving oddly from a certain age onwards: they come early, they come late, the flow changes. It is often assumed that this is already the menopause, but it isn't. It is perimenopause, the transitional stage that comes before it. And understanding the difference matters, because it changes what you can expect and how long it may go on for.

What perimenopause is (and why it isn't the same as menopause)

Perimenopause, also known as the menopausal transition, is the period leading up to the final menstrual period, extending until twelve months after it. It begins in the late reproductive phase, when persistent menstrual irregularity appears, and closes one year after the last period.

The key is telling apart two things that get confused every day. Menopause is a single moment, and it is also defined retrospectively: it can only be confirmed once twelve consecutive months have passed without a period. Perimenopause, by contrast, is not a point but a range: the whole transition that surrounds and leads up to that moment. Put another way, when a woman is going through hot flushes and irregular cycles, she is not in the menopause; she is in perimenopause, on her way there.

The reference framework for organising these stages is the international STRAW+10 consensus, published in 2012, which stages reproductive ageing. According to its criteria, the early transition is identified by a persistent difference of seven or more days in the length of consecutive cycles, and the late transition by intervals of amenorrhoea of sixty days or more. These are objective signs that the cycle is losing its regularity.

Why it happens

The underlying driver is the progressive decline in the ovary's follicle reserve. As that reserve falls, hormone production stops being stable: oestradiol does not drop in a clean, linear way but fluctuates erratically, with ups and downs, while FSH tends to rise and the axis connecting the hypothalamus, pituitary and ovary becomes increasingly dysregulated.

That fluctuation, and not just the decline, is what explains why periods become unpredictable and why symptoms come and go without a clear pattern. It also explains hot flushes: the hormonal swings affect the thermoregulatory centre in the hypothalamus, the part of the brain that regulates body temperature. One important nuance that often gets overlooked: because ovulatory cycles are still possible, pregnancy can still happen during perimenopause.

When it starts and how long it lasts

There is no exact age of onset that holds true for everyone: it varies. What is more consistent is the dynamic of the transition itself. Perimenopause usually lasts several years, counted from the appearance of menstrual irregularity until one year after the final period, although the variability from one woman to another is very wide. The STRAW+10 consensus estimates that its final phase alone, the one with gaps of 60 days or more between periods, lasts on average between 1 and 3 years. For some it will be a short process; for others, a long one.

That is why it pays to be wary of categorical, universal figures about exactly what age it starts or how long it lasts. Talking about several years is a useful reference point, not a fixed rule.

What the evidence says about the symptoms

The most characteristic symptoms are the so-called vasomotor symptoms: hot flushes and night sweats. But reducing perimenopause to hot flushes would be a mistake. Sleep disturbance, mood changes, genitourinary symptoms and vaginal dryness are also common. And something that often comes as a surprise: not all women get hot flushes.

On how long those hot flushes last, the SWAN study provides concrete and striking data. The median total duration of frequent vasomotor symptoms was 7.4 years, with a median persistence of 4.5 years after the final period. In other words, for many women hot flushes do not end with the last period but carry on well beyond it.

That same study also showed that duration depends a great deal on when they start. Women who already had hot flushes in premenopause or early perimenopause were the ones who carried them for longest, with a median of more than 11.8 years. Marked differences also emerged by ethnic background: longer duration in African American women (10.1 years) and shorter in Japanese (4.8) and Chinese women (5.4). The practical takeaway is that there is no single perimenopause experience: the range is enormous.

Treatments and where they stand in Spain

When symptoms affect quality of life, there are options with scientific backing. Menopausal hormone therapy, with oestrogen alone or combined with a progestogen, significantly reduces the frequency and severity of vasomotor symptoms compared with placebo: in the Cochrane review of the oral route, the weekly frequency of hot flushes fell by 75%. The oral and transdermal routes show comparable efficacy. These therapies are authorised and available in Spain.

That said, the decision to use them is an individual assessment. The balance between benefit and risk is not the same for everyone: it depends on age, the time elapsed since menopause, the route of administration and the type of hormone. This is why the risks described in studies such as the Women's Health Initiative, carried out in older, postmenopausal women using specific formulations, cannot simply be transferred to a perimenopausal woman. And on breast cancer it is worth being precise: it is not correct to state that hormone therapy causes it in a simple, universal way; the magnitude of the risk varies depending on whether oestrogen is used alone or in combination, on duration and on each person's profile.

There is also a non-hormonal option: fezolinetant, marketed as Veoza in 45 mg tablets, indicated for moderate to severe vasomotor symptoms. It works as a selective neurokinin 3 receptor antagonist acting on certain neurons in the hypothalamus (the KNDy neurons); according to its summary of product characteristics, it is thought that this helps to restore the balance of the centre that regulates temperature. In Spain it is authorised and available following European approval in December 2023. However, it is not a medicine without precautions: in 2024 the AEMPS issued new pharmacovigilance recommendations because of the risk of liver injury, with liver function tests before starting and periodic monitoring during the first few months; it also carries the inverted black triangle indicating additional monitoring.

Common myths, debunked

• Perimenopause and menopause are not the same thing. Menopause is a single moment confirmed in retrospect, after twelve months without a period; perimenopause is the whole transition before it that encompasses it.
• Oestradiol does not drop all at once in a straight line. It fluctuates erratically, rising and falling, and that is precisely what makes cycles and symptoms so irregular.
• Fertility does not end overnight. Because ovulation is still possible, pregnancy can still happen during perimenopause.
• It isn't all about hot flushes. Many women get them, but others don't; and sleep, mood and genitourinary symptoms count just as much.
• Supplements are not equivalent to hormone therapy. The evidence on phytoestrogens such as isoflavones or red clover is inconsistent and does not allow them to be considered equal to hormone therapy in efficacy.

In short

Perimenopause is a transitional stage, variable and often long, marked by hormonal fluctuation rather than a simple decline. Knowing what defines it, why it happens and what the evidence says helps you put a name to what you are feeling and make decisions more calmly.

If your symptoms are affecting you, there is no need to go through this stage in the dark: proper assessment is medical and individual. What does exist in Spain today is menopause treatment with a prior assessment, a prescription where appropriate from a registered doctor and ongoing follow-up. If you want to know whether it fits your case, you can check whether you are eligible.