Periods every two weeks in perimenopause: what is going on

Why cycles shorten

A menstrual cycle has two halves. The second, after ovulation, is fairly fixed at around fourteen days. The first is variable, and that is where perimenopause acts.

As the pool of remaining follicles shrinks, FSH rises, and the follicles that remain are recruited and mature faster. The first half of the cycle compresses, sometimes to a week or less, and the whole cycle shortens with it.

So a 28-day cycle becoming a 21-day cycle, or a run of bleeds a fortnight apart, is the ovaries responding faster rather than something going wrong structurally.

Telling frequent periods from bleeding between periods

This distinction changes the assessment, and it is worth working out before the appointment.

Frequent periods look like periods: a recognisable flow, lasting several days, with a pattern. Intermenstrual bleeding is different — spotting or light bleeding arriving between proper periods, often brown, often without the usual build-up.

Genuinely frequent short cycles are usually perimenopausal. Bleeding between periods is more likely to be structural, most often a polyp, and is the pattern that warrants a look inside.

When it needs investigating

• Cycles consistently shorter than 21 days from the first day of one bleed to the first day of the next
• Frequent bleeding that is also heavy, which is a fast route to iron deficiency
• Bleeding that never fully stops, rather than arriving in distinct episodes
• Bleeding after sex, at any frequency
• Frequent bleeding starting after a long stretch of settled or absent periods

The iron consequence

Bleeding twice as often means losing roughly twice as much iron, and the body cannot replace it at that rate from diet alone in most cases.

Fatigue, breathlessness, palpitations and hair shedding in someone bleeding every fortnight are more likely to be iron deficiency than hormones. Ferritin is the test that distinguishes them, and it is not included in a standard full blood count unless requested.

When to see someone

Book if it has been going on for more than two or three cycles, and ask for ferritin along with whatever else is arranged. Bring a record of dates — the distinction between short cycles and intermenstrual bleeding is almost impossible to reconstruct from memory.

Any bleeding after sex, or any bleeding after twelve months without a period, needs assessing promptly regardless of the rest.