Two periods in one month

Is it really a second period?

Start with the calendar, because a month can hold two periods without anything having changed. A woman with a regular 28-day cycle whose period starts on 1 March will start again on 29 March. In a 30- or 31-day month that happens every so often, and it says nothing about perimenopause.

What matters is the gap, counted from the first day of one period to the first day of the next. The NHS calls a cycle irregular when that gap is shorter than 21 days or longer than 35. The American College of Obstetricians and Gynecologists (ACOG) puts a normal cycle at 24 to 38 days. So a 25-day cycle that lands you with a period at both ends of a month is within the normal range, even if it feels new to you.

The second thing to sort out is whether the second bleed was a period at all. A full flow that builds and tapers over several days reads as a period. A few days of brown or pink staining, or a smear on the paper, is spotting, and spotting between periods has its own list of causes and its own advice. Our guide to spotting between periods in perimenopause goes through them. Writing down the flow on each day settles the question quickly.

Why periods come closer together in your 40s

A cycle has two halves. The first, the follicular phase, runs from the start of a period until an egg is released. The second, after ovulation, stays fairly steady in length. Almost all of the shortening seen with age happens in the first half.

A 1996 study of perimenopausal hormone patterns measured hormones in daily urine samples and found that women aged 47 and over who were still having periods had follicular phases of about 11 days, compared with about 14 in women aged 19 to 38. Their follicle-stimulating hormone (FSH) levels were higher too. A textbook chapter on the endocrinology of perimenopause by the same research group explains the link: as the ovaries respond less well, the body produces more FSH, the egg-carrying follicle develops faster, and ovulation comes sooner. The chapter puts the result at a gap between periods about 3 days shorter in most women, and notes that this shortening appears before hormone changes show up clearly in blood tests.

Three days off a cycle is enough to turn a 27-day pattern into a 24-day one, and a 24-day cycle will regularly fit two periods into one calendar month.

Short cycles and where you are in perimenopause

The STRAW+10 staging system, which researchers and clinicians use to describe reproductive ageing, lists shorter cycles as the change that marks the late reproductive years, before the menopausal transition has formally begun. In other words, a period twice a month in your early 40s often comes earlier than the symptoms people associate with menopause, such as hot flashes.

Cycles rarely stay short for good. STRAW+10 dates the start of the early transition to cycles whose lengths differ by 7 days or more from one to the next, again and again. The textbook chapter calls perimenopausal cycles "irregularly irregular" and says there is no orderly progression between short and long ones. A 22-day cycle can be followed by a 40-day one. Later, gaps of two months without a period become common, and the article on irregular periods in perimenopause follows the pattern through to skipped months. For the ages these stages usually fall at, see when perimenopause starts and how long it lasts.

When to see a doctor about periods twice a month

ACOG says changes in bleeding are expected in perimenopause and should still be discussed with a doctor. Most cycles that drift a few days shorter can wait for a routine appointment. I would book one sooner, rather than wait for the next check-up, if any of these apply:

• your periods keep starting less than 21 days apart, cycle after cycle
• each period lasts longer than 7 days, which the NHS gives as a reason to see a GP
• the bleeding is heavy: a pad or tampon soaked every hour or two, clots the size of a 10p coin or a US quarter, or bleeding through to clothes or bedding (see heavy periods and blood clots in perimenopause)
• you feel tired all the time or short of breath, which the NHS lists among the signs that heavy periods need checking
• you bleed between periods or after sex
• you bleed at all after 12 months without a period

Frequent periods add up. Bleeding every three weeks means more days of blood loss across a year than bleeding every four, and if each period is also heavy, iron levels can fall. That is why tiredness belongs on the list, and why a doctor may suggest a blood count.

Other causes of periods close together

Perimenopause is the likeliest explanation after 40, but the NHS names several other causes of irregular periods, and some of them shorten the cycle:

• hormonal contraception, including the progestogen-only pill, the injection and the hormonal coil, which can also bring bleeding at odd times
• stress and anxiety, a large change in weight, and very heavy exercise
• thyroid problems, which can make periods irregular or heavier (our comparison of perimenopause and thyroid symptoms sets out what points each way)
• polycystic ovary syndrome

ACOG adds causes inside the womb, such as polyps and endometrial hyperplasia, a thickening of the lining. A bleed that arrives on an odd schedule is sometimes one of these rather than a true period, which is why a doctor wants to hear about a change in bleeding instead of having you wait it out.

Pregnancy is still possible in perimenopause, since ovulation can carry on until the last period. If there is any chance you could be pregnant, take a test. The NHS treats a missed period followed by unusual bleeding and tummy pain as urgent, because it can be an ectopic pregnancy.

How to tell from your own dates

A month with two periods is hard to judge on its own. Three or four months of dates show whether your cycles have become shorter for good, are swinging between short and long, or were thrown off just once. For each period, note:

• the day bleeding started, which counts as day 1
• how many days it lasted
• the flow on each day, from spotting through light and medium to heavy
• any bleeding that came between periods or after sex

Then count the days from one day 1 to the next. Mark every gap under 21 days, and every cycle that is a week longer or shorter than the one before. Those two marks are what a doctor will look for first. For women over 45, the NICE guideline on menopause says perimenopause is usually recognised from symptoms and cycle changes without hormone tests, so your record carries real weight. Our printable symptom diary and appointment checklist has rows for flow and spotting next to your other symptoms.

My take

If you are in your 40s and your periods have moved from every four weeks to every three, with bleeding that is otherwise the same as always, I'd treat it as the ordinary early drift it usually is, start writing down the dates, and raise it at your next appointment. I'd judge it differently in two cases. One is cycles that keep falling under 21 days. The other is any heavier, longer or in-between bleeding, which I would book in for that week. A short list of start dates answers most of what a doctor asks about the change, and it is easier to bring than to reconstruct from memory.

Related reading

The periods and bleeding topic collects every piece on cycle changes, from the first short cycles to the last period. For the wider set of symptoms that can come alongside them, start with perimenopause symptoms and how to track them.