High Before Your Period, Low at the Start: The Blood Sugar Pattern Nobody Warned You About (Type 1 Diabetes)
If your blood sugars run high the week before your period and then crash low when it starts, you’re not imagining it. It’s one of the earliest signs that perimenopause is starting to affect your Type 1 diabetes, and almost nobody warns us this is coming.
In Type 1 diabetes, blood sugars often run higher in the week before your period and drop lower when it starts because progesterone rises in the luteal phase and contributes to insulin resistance, then drops sharply at the start of your cycle, increasing insulin sensitivity. This pattern often becomes more pronounced in your late 30s and 40s as perimenopause begins.
The First Time I Caught the Pattern
For years, I had days where my blood sugars made no sense.
I’d eaten this meal a hundred times. I’d taken the same insulin. I’d done everything the same. And the number on my CGM was telling me a completely different story than yesterday’s number had.
The first thing I did was blame myself. Did I miscount the carbs? Was the insulin bad? Did I miss a bolus? I’d run through the checklist and find nothing.
Then I started noticing something.
The week or two before my period, my blood sugars ran consistently higher. My insulin needs went up. The pre-bolus timing that usually worked stopped working. I’d correct, and it would barely move.
Then my period would start — and the floor would drop out. Lows I had to chase. CGM alarms in the middle of the night. Insulin sensitivity that suddenly felt like I was a different person.
It wasn’t random. It was hormonal.
What’s Actually Happening in Your Body
Estrogen and progesterone don’t just fluctuate during your reproductive years — they directly affect insulin sensitivity. And when you’re managing insulin manually with Type 1 diabetes, even a small shift can send your numbers somewhere you weren’t expecting.
Here’s the simplified version of what’s going on across the cycle:
The luteal phase (the week or two before your period): Progesterone rises. From what I’ve read and experienced, progesterone is thought to contribute to insulin resistance, which means your body needs more insulin to do the same job. Many T1D women notice they need more insulin during this stretch and that corrections feel less effective than usual.
The start of your period: Progesterone drops quickly. Insulin sensitivity often increases — sometimes dramatically. The same insulin doses that barely held you steady three days ago can suddenly send you low.
The follicular phase (the first half of your cycle): Things tend to feel more stable, though estrogen is still moving. This is often the stretch where T1D management feels most predictable for women who track this pattern.
In a body without Type 1 diabetes, these hormonal shifts get absorbed quietly. The pancreas adjusts insulin output in real time, and the woman barely notices. When you’re dosing insulin manually, those same shifts hit the system without that automatic buffering — and the result is a much bumpier ride.
Why This Pattern Often Gets Worse in Your Late 30s and 40s
Here’s the part nobody warned me about.
In your 20s and early 30s, even if your cycle was affecting your blood sugars, the pattern was usually subtle enough to absorb. Hormones were predictable. Cycles were regular. The variability was there, but it didn’t feel chaotic.
Then perimenopause begins, often quietly, often years before periods become irregular. Estrogen and progesterone start fluctuating more dramatically. The patterns that used to be subtle become harder to ignore. Insulin needs that used to shift by small amounts start swinging more widely. Cycles that used to be reliable become less so.
If you’re in your late 30s or 40s and your blood sugar variability around your cycle has gotten noticeably worse, this is a real and known phenomenon. It’s not that you’ve gotten worse at managing your diabetes. The hormonal environment has changed.
You can read more about why blood sugar feels harder in midlife generally in Why Blood Sugar Feels Harder in Perimenopause (and it’s not just you) This post is the deeper look at one specific piece of that puzzle.
What This Looks Like Day to Day
If you’re not sure whether your blood sugars are tracking your cycle, here are the patterns to watch for:
- Higher fasting numbers in the 7–10 days before your period
- Increased insulin needs across the board during that same window
- Stubborn highs that don’t respond to corrections the way they normally would
- Pre-bolus timing that suddenly feels off
- Sharp shift to lows starting the day before or the day your period begins
- Overnight numbers becoming less predictable, especially in the late luteal phase
- A sense that you’re “chasing” insulin during one stretch and “chasing” lows during the other
You may not see all of these. You may see most. The thread connecting them is the underlying hormonal fluctuation, and once you know to look for it, the patterns get a lot easier to spot.
When the Pattern Doesn’t Show Up (and Why That’s Also Normal)
Here’s the part most articles leave out.
You’ll start tracking, expecting to see the clean “high before, low at the start” pattern — and some months it just won’t show up. Or it’ll show up backwards. Or your numbers will be all over the place in ways that don’t match where you are in your cycle at all.
That’s not a tracking failure. That’s perimenopause.
The textbook pattern is based on a regular ovulatory cycle with a predictable progesterone surge. In perimenopause, more and more cycles become anovulatory — meaning you don’t ovulate that month, and the progesterone rise that drives the late-cycle insulin resistance simply doesn’t happen. You can also have cycles where estrogen spikes much higher than usual without progesterone rising to balance it, which produces a totally different blood sugar pattern.
Then there’s everything else stacked on top.
Cortisol is a major driver of insulin resistance, and in perimenopause, cortisol patterns get dysregulated independently of where you are in your cycle. A bad night of sleep, a stressful week, a hard workout — any of these can push your numbers higher even in a phase that “should” be sensitive. Inflammation tends to run higher in perimenopause too, and inflammation affects insulin sensitivity directly. If you have any thyroid history, that’s another variable that can mask or scramble the cycle pattern entirely. And the deep sleep that used to help regulate your morning numbers is genuinely changing in ways your body didn’t have to deal with at 30.
So if you’re tracking carefully and the pattern still doesn’t make sense some months — you’re not doing it wrong. You’re seeing perimenopause for what it actually is. Multiple variables shifting at the same time, with no single one fully explaining what your CGM is showing you.
That’s also why community matters so much in this season. Other women living this fill in gaps that even the best research can’t, because we’re all running slightly different experiments on ourselves and noticing things one at a time.
How to Start Tracking This for Yourself
The fastest way to confirm whether your cycle and your blood sugars are connected is to start tracking them side by side.
You don’t need a fancy system. What I’d recommend:
Track your cycle. Mark day one of your period as the start of a new cycle. Note any spotting, irregular bleeding, or skipped cycles.
Pay attention to your CGM data by cycle phase. Look at your time in range, your average glucose, and your total daily insulin in the week before your period versus the week of and the week after. After two or three cycles, the pattern usually becomes visible.
Note how corrections behave. Are they working the way they normally do? Or are you needing more insulin to bring numbers down in certain weeks?
Trust your symptoms, not just the numbers. If you feel more insulin resistant — sluggish, irritable, “stuck high”, that’s data. Write it down.
After a couple of months, you’ll start seeing the rhythm. And once you see it, you stop blaming yourself for the variability.
What To Do With What You Find
I want to be careful here. I’m not a clinician, and insulin adjustments need to be made with your diabetes care team. But here’s what I’d suggest as a general approach once you’ve spotted your pattern:
Bring it to your endocrinologist. Ask specifically about adjusting insulin-to-carb ratios, basal rates, or sensitivity factors during the late luteal phase. Some T1D women run on different settings during that week than the rest of the month. A good endo will engage with this question.
Ask about temporary basal increases or decreases. If you’re on a pump or hybrid closed-loop system, temporary basal adjustments can be a useful tool around predictable hormonal shifts. Your care team can help you figure out timing and amount.
Don’t expect perfection. Even with adjustments, the rollercoaster doesn’t completely flatten. The goal is fewer extremes and less self-blame, not a perfectly steady CGM line. Anyone telling you otherwise hasn’t lived this.
Watch for cycle changes themselves. If your periods are becoming irregular, longer, heavier, or skipping, that’s its own conversation with your OB-GYN. Don’t accept “it’s just stress” as the answer if your gut is telling you something more is going on.
The Bigger Picture
The cycle-blood sugar connection is one of the very first signs that perimenopause is beginning to affect your Type 1 diabetes. And it’s just one piece of a much larger picture — one that includes sleep changes, mood shifts, stubborn weight gain, fatigue, and a long list of symptoms that often arrive without anyone naming what’s happening.
I wrote a free guide called What Nobody Told Me: How I Finally Understood What Perimenopause Was Doing to My Type 1 Diabetes because I lived through years of confusion before I had any of this language. It walks through the symptoms, the science in plain terms, the moments that scared me, and what actually helped — including the setbacks. If this post resonated, the guide goes much deeper.
Use the form below to grab my free guide. I’ll send it straight to your inbox.
You Are Not Failing
If your blood sugars have started behaving like they belong to someone else around your period, please hear me on this:
You are not failing. The variables changed.
Your body is doing what bodies do at this stage of life. Your hormones are shifting. Your insulin sensitivity is moving with them. The numbers on your CGM are reflecting a real, physiological change — not a personal one.
The good news is that once you can see the pattern, you can start working with it instead of being blindsided by it. Your strategy can change with your body. And it should.
Frequently Asked Questions
Why do my blood sugars run high the week before my period?
In the luteal phase, progesterone rises and is thought to contribute to insulin resistance, which means your body needs more insulin to do the same job. For women with Type 1 diabetes, this often shows up as higher fasting numbers, stubborn highs, and corrections that don't work as well as usual.
Why do I crash low when my period starts?
When your period begins, progesterone drops sharply and insulin sensitivity often increases. If your insulin doses haven't adjusted to match, the result can be unexpected lows in the first day or two of your cycle.
Does this pattern get worse in perimenopause?
For many women with Type 1 diabetes, yes. As estrogen and progesterone start fluctuating more dramatically in your late 30s and 40s, the cycle-related blood sugar swings often become more pronounced and harder to ignore.
Should I adjust my insulin based on my cycle?
That's a conversation for your diabetes care team. Some women adjust insulin-to-carb ratios, basal rates, or sensitivity factors during specific phases of their cycle, but those changes need to be made carefully and with clinical guidance.
How long does it take to see the pattern?
Most women start spotting it after tracking two to three full cycles alongside CGM data. The patterns become clearer the longer you track.
What if my blood sugars don't match the cycle pattern at all?
That's also common, especially in perimenopause. Anovulatory cycles (months where you don't ovulate) don't produce the typical progesterone surge, so the late-cycle insulin resistance doesn't show up. Cortisol, inflammation, sleep changes, and thyroid shifts can also mask or override the cycle pattern. If your data isn't matching the textbook description, you're not tracking wrong — perimenopause is messier than the simple version makes it sound.
Key Takeaways
- Higher blood sugars in the week before your period and lows at the start are common in Type 1 diabetes and connected to hormonal shifts in progesterone
- This pattern often becomes more noticeable in your late 30s and 40s as perimenopause begins
- Tracking your cycle alongside your CGM data is the fastest way to see the pattern for yourself
- Insulin adjustments based on cycle phase are possible but should be made with your diabetes care team
- This is a normal hormonal response to a real physiological change — not a sign you're managing your diabetes wrong
If you want the deeper picture of everything perimenopause does to a Type 1 body — and what actually helped me feel like myself again, grab my free guide What Nobody Told Me (opt-in link to be added once Kartra page is live).
You are not crazy. And you are absolutely not alone.
