Show notes
This is episode two of Fat Fertility 101, and we’re getting practical about how to actually find your fertile window. In the last episode, we covered how the menstrual cycle works. Today we’re diving into the details of figuring out when your body ovulates so you can time sex or insemination for the best chance of getting pregnant.
For folks with irregular cycles (especially those with PCOS), this can feel like stabbing in the dark. So we’re going through practical tools to help you figure this out, even if your cycles are all over the place.
In this episode, we cover:
How to calculate your fertile window if you have relatively consistent cycles, using your last three cycles to find patterns
What to do when your cycles are completely irregular and tracking feels impossible
Three different ovulation tracking methods (BBT, OPKs, and devices like Mira) with honest pros and cons of each
Why ovulation predictor kits sometimes don’t work for folks in bigger bodies and what’s actually happening
Why trusting your body’s signals matters more than any pee stick reading
Useful Links:
The Full Story
If you’re trying to conceive and you have regular, predictable cycles, there are formulas and apps and all sorts of tools that work brilliantly for tracking ovulation. But if you’re like me, with PCOS and cycles that can stretch past 100 days, the cheerful advice to “just track your cycle” feels like a sick joke.
You’re basically stabbing in the dark, hoping to somehow hit that tiny fertile window when you have no idea when it’s coming.
I know how frustrating this is because I’ve navigated it myself. And what I’ve learned is that even when your cycles are wildly irregular, there are practical ways to gather information about your body. Any data is better than no data, and even small bits of information can help you feel less lost in the process.
Calculating Your Fertile Window with Regular-ish Cycles
If your cycles are relatively consistent, even if they’re not the textbook 28 days, here’s how to work out your fertile window.
I recommend looking at your last three cycles. The more data you have, the better, but three is a good starting point. For each cycle, note how long it was and roughly what day you ovulated (if you know).
If you don’t know when you ovulated yet, don’t worry. You can start gathering this information moving forward, cycle by cycle. There’s no rush. We’re just building awareness about how your body works.
Here’s the important thing to remember about data: each individual data point is just one snapshot in time. We can’t really assess anything based on one snapshot. What we’re looking for are trends over time, patterns that emerge when you average things out over at least three months.
Let’s walk through an example. Say you ovulated on day 15 in one cycle, day 18 in another, and day 17 in a third. Add those together (that’s 50), divide by three, and you get an average of about day 17.
Your fertile window is five days before that average ovulation day, plus the day itself. So in this example, it would be days 12-17.
This is just a rough estimate, a guide to help narrow things down. During this window, it can be really helpful to stay in touch with any signs or symptoms that coincide with ovulation for you. Things like changes in cervical mucus, increased sex drive, cramping, more energy. Noticing these patterns helps you recognise when you’re approaching that fertile window.
When Cycles Are Completely Irregular
I know what some of you are thinking, because I’ve thought it myself: that’s all well and good, but my cycles aren’t every month. They’re 100-plus days. This doesn’t work for me.
I hear you. I’ve been there. Here’s what I’ve found most helpful when navigating irregular cycles with PCOS.
Find some way to track information, whether that’s pen and paper or an app on your phone. But this isn’t a long, elaborate journaling session. This is 30-60 seconds each day tracking a few useful things:
How’s your energy today, maybe on a scale of 0-10? Did you feel particularly anxious? Notice any unusual aches or pains? Was your libido higher than usual? Did you notice changes in your cervical mucus?
Don’t try to track everything. Just pick a few things that feel manageable and stick with those.
Then when you do get your period (hopefully at some point), count back 10-14 days. Look at what was going on during that time, because we know that for most folks, menstruation comes 10-14 days after ovulation. This gap usually stays consistent even with irregular cycles.
When those symptoms pop up in future cycles, you can recognise them. “Oh, this is what was happening 10-14 days before my last period. Could this be ovulation?” It prompts you to pay a bit more attention during those few days and start recognising patterns.
It’s not perfect. It’s not concrete. But when you’re feeling like you’re completely in the dark, any data can be genuinely helpful.
Three Tracking Methods: The Honest Pros and Cons
There are three main ways people track ovulation beyond just noticing body signs. Let’s go through each one so you can decide if any might be helpful for you.
Method 1: Basal Body Temperature (BBT)
This is where you use a thermometer to measure your temperature every day when you first wake up. You can do it in your mouth, armpit, or vaginally. It needs to be done at roughly the same time each day with an accurate thermometer (one that goes to two decimal places, because the shift is quite small).
What happens is your temperature follows a baseline, then just before you ovulate it drops slightly, then it rises due to progesterone. This is called a biphasic shift (bi meaning two, phase meaning shift). The second part of your cycle, the luteal phase, is when your temperature is slightly higher.
This pattern confirms that you’ve ovulated. When you see that dip and then the rise, you know your body released progesterone, which generally means ovulation happened.
Pros: Really low cost (you just need a thermometer), and there’s potential to gather lots of information about your hormones. You can use it simply to confirm ovulation with that biphasic pattern.
Cons: Quite a steep learning curve to use the method well. You need to take it at the same time every day to get accurate readings, which can be frustrating depending on how you are with routines. It’s also prone to variability. Lots of things can affect your temperature: waking in the night, drinking alcohol before sleep, poor sleep quality, being unwell. It’s about looking at trends rather than taking any single reading as gospel.
The main limitation for predicting ovulation is that you can only confirm it happened afterwards. It doesn’t give you advance warning that ovulation is coming, which can be tricky with irregular cycles.
If you want to really dive into this method, the book is “Taking Charge of Your Fertility” by Toni Weschler. It’s a chunky book but will teach you everything about tracking, charting, and understanding the different patterns.
Method 2: Ovulation Predictor Kits (OPKs)
These are pee sticks that measure your LH (luteinising hormone), which surges just before ovulation. Unlike BBT, these try to predict ovulation rather than confirm it afterwards. They can give you that bit of warning that ovulation is about to happen.
Pros: Relatively low cost per stick (you can buy them in bulk), and they can give great confirmation when they work.
Cons: Sometimes they don’t work for folks in bigger bodies. This isn’t something that’s widely publicised or often discussed, but it’s really important to understand.
For folks in bigger bodies, our LH can be a little bit lower. We still get that peak, everything’s happening normally, but it appears lower. The stick is set to detect a specific threshold (probably because they were developed and tested on folks in smaller bodies). If we don’t reach that threshold, it doesn’t mean anything is wrong. It just means our level is lower and doesn’t show up on the stick.
Looking at the research, there isn’t an obvious reason why this happens. But what makes most sense to me (and it’s the same with pregnancy tests) is that we have the same level of hormone, but at a lower concentration because we have higher body mass and a higher volume of fluid in our bodies. The concentration of that hormone being measured is lower, so it isn’t detected as easily.
This can be really frustrating if you’re not getting confirmation of ovulation, especially when you’re experiencing all the signs and symptoms. My main takeaway: trust your body over a pee stick. Just because it doesn’t show you’ve ovulated doesn’t necessarily mean that’s true.
That said, we’re not a monolith. OPKs may well work for you, and they’re relatively low cost if you want to give them a try.
Method 3: Hormone Tracking Devices
These are small at-home devices (Mira is one example, but there are others) that give you actual hormone level readings rather than just a yes/no answer.
You pee on a stick and the device shows your actual hormone levels. For me with PCOS, this was brilliant because I could see I did have an LH peak, it was just very low. I could see the baseline, see the peak, and recognise it was my LH peak even though the level was too low to show up on regular OPKs.
These devices now often measure multiple hormones: LH, FSH, progesterone. You can track everything in one device, which can be really helpful if you’re concerned about your hormone levels throughout your cycle. It gives you accurate data over time rather than just one snapshot from a blood test.
Pros: You get accurate data and actual numbers. It can both predict and confirm ovulation. You don’t have to use it at the same time every day (unlike BBT), and it will actually prompt you to test again later if it thinks that would help catch the peak.
Cons: It’s expensive. The device costs a couple of hundred pounds, and you need to keep buying the pee sticks. It’s an ongoing cost to consider.
All tracking methods can start to feel obsessive sometimes. It can get to a point where it’s taking over your brain, where you’re putting too much weight on individual numbers. The question is always: what’s best for meeting your needs and supporting your mental and physical health?
A Note About LH Levels in Bigger Bodies
If you do use something like Mira and find your LH level is quite low, that’s absolutely okay. A lot of research shows that the “normal” reference ranges were tested on folks in what would be considered a “normal” body size.
Because our bodies are bigger, we have a higher total volume of blood. The concentration of that hormone can be lower even though the total amount is fine. That doesn’t mean anything is wrong. It just means our bodies are different.
Our normal reference ranges are different, but that’s not considered in a fatphobic healthcare system that wouldn’t think to account for body size differences. Don’t worry if you get a low peak. It absolutely still means you’re ovulating. Your body is just different, and that’s completely okay.
What to Remember
If you have relatively regular cycles, start tracking them. Look back at your last three cycles if you can and calculate that average ovulation day. Your fertile window is five days before that plus the day of ovulation.
If you have irregular cycles, start tracking how you feel each day. Just quick snapshots. Build up that data so when you do get your period, you can look back and start spotting patterns.
If you want to use tracking tools, pick the one that works for your budget and feels sustainable. There’s no perfect method. The best method is the one you’ll actually do. And it doesn’t matter if you can’t do it consistently. Just do what you can. Any data is good data.
In the next episode of Fat Fertility 101, we’re tackling some important myth-busting, because there’s so much rubbish information out there about fat fertility. I’m going to set the record straight about what the research actually says.
Ready for personalised support navigating all of this? Whether you’re drowning in tracking data and don’t know what to do with it, need help figuring out what tests to ask for, or want to create a plan that actually works for your brain, that’s exactly what we work through in my Fertility Roadmap sessions. It’s a completely shame-free, weight-neutral space where you’ll feel validated, supported, and heard.
