Cycle Changes in Perimenopause: What's Normal and What's Not

For most of their adult lives, women develop a working knowledge of their cycle. The rough length, the character of each phase, what feels normal for them. Perimenopause disrupts all of that. The cycle that was reliably 28 days becomes 23, then 35, then absent for six weeks. The period that was moderate becomes flooding, or dwindles to a few days of spotting. Something that felt stable and known is suddenly unpredictable.

This is one of the earliest and most common signs of perimenopause. And because it happens at the level of a system women thought they understood, it can feel particularly unsettling.

Why the cycle changes

The menstrual cycle is regulated by a hormonal conversation between the brain and the ovaries. The pituitary gland sends signals - follicle-stimulating hormone (FSH) and luteinising hormone (LH) - and the ovaries respond by developing follicles, releasing eggs, and producing oestrogen and progesterone in a coordinated rhythm.

In perimenopause, the ovaries become less responsive to those signals. The follicle pool is depleting. Ovulation may happen inconsistently - sometimes earlier than usual, sometimes later, sometimes not at all in a given cycle. When ovulation doesn't happen, progesterone isn't produced (since progesterone is mainly produced by the corpus luteum after ovulation), and the hormonal balance of the cycle is disrupted.

This is why the cycle can become unpredictable in multiple ways simultaneously: different lengths, different hormone levels, different amounts of uterine lining building and shedding.

What tends to change, and in what order

The first change is often in cycle length. Cycles that shorten slightly - becoming 24 or 25 days rather than 28 - are a commonly reported early sign of perimenopause. This happens because the follicular phase (the first half of the cycle, leading to ovulation) shortens as follicle development becomes less consistent.

Later in perimenopause, cycles typically become more variable rather than just shorter. The interval between periods may lengthen. Gaps of six, eight, twelve weeks become possible. Periods may become heavier - sometimes dramatically so - as cycles with no ovulation allow the uterine lining to build for longer before shedding.

Periods becoming lighter or shorter is also common, particularly as the transition progresses toward its end.

What is within the range of normal

Cycle changes during perimenopause exist on a wide spectrum, and most of the following fall within the range of the transition rather than indicating something concerning:

• Cycles varying by seven or more days (where they were previously regular)
• Occasional cycles that are significantly shorter or longer than the personal average
• Cycles that are missed entirely and then return
• Periods that are heavier or lighter than usual
• Changes in the character of premenstrual symptoms

These are all part of the perimenopausal picture.

What warrants a conversation with a doctor

Some changes during this period are worth discussing medically, not because perimenopause has necessarily stopped being the cause, but because other causes should be ruled out.

Bleeding between periods warrants assessment. Very heavy bleeding - soaking through a pad or tampon every hour for several hours - warrants prompt assessment. Periods returning after twelve or more months of absence warrant assessment (once twelve months of no periods have passed, a return of bleeding should always be investigated). Bleeding after sex should always be discussed with a GP.

These symptoms are often still hormonally driven, but they can also be signs of conditions that need attention - including polyps, fibroids, or in some cases endometrial changes - and a proper evaluation is important.

Contraception and fertility during perimenopause

An important and often surprising point: irregular cycles during perimenopause do not mean that pregnancy is impossible. Ovulation can still occur, unpredictably. Women who are sexually active with a partner capable of causing pregnancy should discuss contraception with their GP during perimenopause, as it remains relevant until menopause is confirmed (twelve consecutive months without a period) and typically for one to two years beyond that in women under 50.

The irregularity of the cycle makes it impossible to rely on cycle-based contraception during this phase.