PMS or Perimenopause? How to Tell the Difference in Your Late 30s and 40s

You’re 39. You run a team, you run a household, and lately you also run on about four hours of sleep. You wake at 3 a.m. with your heart going for no reason at all. Your breasts have been sore for two solid weeks. You’re bloated in jeans that fit fine in March, and your period showed up twice in October, then skipped November entirely.

So you do the responsible thing. You make an appointment and you explain all of it. And you get told you’re too young for this to be hormones, handed a prescription for birth control pills or an antidepressant, and sent on your way in seven minutes.

You sit in the parking lot afterward, frustrated, because you know your body and you know that whatever this is, it isn’t in your head.

Here’s what I want you to know. If you’re in your late 30s or early 40s and any of that sounds like your life, you may be in perimenopause. Not approaching it. In it.

Why are we talking about this in September?

Because September is Perimenopause Awareness Month, and I’d argue that fact is the whole problem in miniature.

The National Menopause Foundation and the perimenopause community Perry launched the first one in 2024, specifically to get this phase talked about on its own terms instead of as a footnote to menopause (National Menopause Foundation, 2024). The following year, twelve governors signed official proclamations recognizing it, North Carolina among them (National Menopause Foundation, 2025). That’s genuinely good news, and I’ll take it. It’s also a little bit of an indictment. We don’t build awareness campaigns around conditions that get taken seriously in the exam room. We build them for the ones where women spend years being told it’s stress, it’s anxiety, it’s your age, it’s nothing.

This is also why I wrote my book in 2019 when no one was talking about perimenopause. Thankfully, it gets more attention now, but there’s still too much “surprise” involved for many women. I wrote the book to be the “What to Expect When You’re Expecting” for perimenopause. Go to your favorite bookseller and grab a copy (and yes, there’s an audio version).

So since we’re here, let’s use the month for something useful. Here’s how to tell what’s actually going on in your body.

What exactly is perimenopause?

Perimenopause is the stretch of time leading up to your final period. It’s not a single event and it’s not a diagnosis. It’s a transition, and for most women it runs somewhere between four and eight years. It can begin as early as your mid to late 30s, right in the thick of raising kids or thinking about having more, while your ovaries are quietly drawing up their retirement plan.

The official starting line is less dramatic than you’d expect. Researchers mark the beginning of the transition when your cycle length starts varying by seven days or more from one cycle to the next, and keeps doing it (Harlow et al., 2012). That’s the criteria. Your first real sign of perimenopause might be nothing more exciting than a 28-day cycle that came at 24 days, then 33.

Why do perimenopause symptoms start so early?

Because the hormone that falls first is the one almost nobody thinks to look at: progesterone.

You only make meaningful progesterone after you ovulate. Starting in your late 30s, you begin having cycles where you don’t ovulate at all, or where you ovulate late and the back half of your cycle is short. You still get a period, so on paper everything looks normal. But progesterone in those cycles is low (Prior & Hitchcock, 2011).

Estrogen, meanwhile, is not gently tapering off into the sunset. This is the part that surprises women most. During perimenopause estradiol swings erratically HIGH, and in roughly a third of cycles there’s a surge of estrogen that shows up out of nowhere in the second half of the cycle (Prior & Hitchcock, 2011). So you’re not low on estrogen yet. You’re low on the hormone that balances it.

That combination, too little progesterone against too much unopposed estrogen, explains most of the vague and maddening early symptoms: the insomnia, the breast tenderness, the bloating, the anxiety that arrives at 3 a.m. with no invitation, the periods that get heavier or closer together.

Testosterone drifts down slowly with age, but estrogen eventually falls faster, which shifts the ratio between them. That’s where the chin hairs come from.

And if you happen to be under real stress at the same time (a demanding job, teenagers, aging parents, or all three at once), cortisol joins the party and turns the volume up on everything.

Is it PMS or perimenopause?

This is the question I get more than almost any other, and the answer comes down to timing, not intensity.

Track your symptoms against your cycle for two or three months. Mood swings, cravings, sore breasts, irritability, bloating, sleep. Write down the day each one starts and the day it stops. Then look at the pattern:

• If symptoms show up in the week or two before your period and clear out once bleeding starts, that’s the PMS pattern.
• If symptoms are present most of the month, or they don’t lift when your period arrives, or they’ve changed character over the last couple of years, you’re most likely looking at perimenopause.

A study that clustered more than 145,000 symptom logs found the perimenopausal pattern is a blend rather than a swap: women in the transition hang onto the cycle-linked symptoms they’ve always had and stack new ones like hot flashes and night sweats on top (Aras et al., 2025). So it’s often not PMS versus perimenopause. It’s the PMS you know, plus new company.

One more thing worth knowing. If you’ve always had rough PMS or PMDD, the early transition can crank it up (Sander & Gordon, 2021). That isn’t you suddenly failing to cope with something you used to handle. That’s a hormone environment that changed underneath you.