Old habits die hard

As you may recall from my last post, the current plan is IVF homework this month; stims and ER next month; detox in July; and FET in August. Not wanting to waste a single egg (what if it’s my only good one left?!), I naturally asked Dr. Y for permission to try ‘the old-fashioned way’ this month. Ever the gentleman, Dr. Y refrained from sharing his thoughts (Why not just enjoy the break? Don’t you realize how low your chance of success is?), and he politely said that would be fine.

So I pulled my BBT thermometer, pen, and a blank chart out of the nightstand drawer, and dug around in the bathroom cupboard for my ClearBlue Fertility Monitor and a stash of test sticks. (Okay, so I may have used the phrase ‘the old-fashioned way’ a tad liberally…) I had skipped all this during our IUI cycle thinking it would be a relief not to have to trouble myself with the morning routine, but I actually ended up regretting it. Throughout the cycle I found myself missing all that precious data! I wanted answers:

  • Would the Menopur injections cause a ‘peak’ reading on the monitor?
  • What about the hCG trigger shot?
  • How long after the trigger shot did my BBT rise?
  • Did the progesterone suppositories cause a higher BBT than usual?

I don’t know! And that bugs me a little bit.

Anyway, I’m back to collecting my precious data this month (and probably will through IVF too, because, why not?!)

And to everyone who wondered how the ClearBlue Fertility Monitor works, the rest of this post is for you. (Wait, nobody is wondering that? In that case, read this hilarious post by Stupid Stork instead…)

Still here?

So, the ClearBlue Fertility Monitor…

Like OPKs, the ClearBlue Fertility Monitor (or CBFM for short) monitors the levels of hormone in my urine. While OPKs detect luteinizing hormone (LH) that surges 24-48 hours prior to ovulation, CBFM detects both LH and estradiol (E2). E2 rises a bit sooner, and a bit more gradually than LH, which means the CBFM can give me more advance warning before ovulation. (This makes it easier to have some semblance of romance in this whole TTC thing. I can say ‘It’s been awhile since we’ve gone out; let’s make Wednesday a date night,” instead of “Wake up! Sexytime! Now! NOW!”)

Each morning starting on CD6, I POAS, cap the little stick, and snap it into the appropriate slot on the monitor. The monitor waits 3 minutes for the stick to develop and then shines a little red light on the stick ‘reading’ the result. For the scientists reading this, I assume the monitor works like a visible absorbance spectrophotometer; I’m looking forward to taking it apart to investigate once I’m sure I don’t need it anymore…

Anyway, after reading the stick, the monitor displays one of three possibilities:

  • Low: low E2 and LH levels. You can have sex today for fun or romance, but you can’t in good conscience use TTC as an excuse.
  • High: high E2 but low LH levels. You can use TTC as a pretty good excuse to have sex today.
  • Peak: LH surge. Ovulation is imminent. Sex today is pretty much required.

The sticks (which you have to purchase separately) look a lot like OPK sticks. But there’s no ‘control’ line – just one line for E2 and one for LH. And the color changes (particularly the E2 color change) are definitely more subtle than for OPKs – hence the need for the monitor to read the result.

I tried, unsuccessfully, to figure out the chemistry (or biochemistry) behind how CBFM works. I imagine that the LH line works using antibodies in a way similar to what I described in this post about how HPTs work, but I don’t know for sure. The mechanism for detecting E2 has to be somewhat different since (a) it’s not a protein hormone, and (b) the E2 line gets lighter as E2 levels increase, instead of darker.

Anyway, here’s a figure showing my monitor & corresponding test sticks for each possible fertility reading:


Note the cute little egg symbol on the display for ‘peak’ fertility.

I color coded the hormone labels in the figure above to match this diagram I found on the interwebs showing how the menstrual hormones rise and fall at varying stages in a cycle. Note the gradual estradiol rise (blue), peaking a day or two before the LH surge (green):


Incidentally, while doing a Google image search for LH and estradiol levels, I found the coffee mug above right, which I would want…except that I don’t think I’m gutsy enough to use it in public. They also sell a hat…

Leave a comment


  1. I love the CBFM. I didn’t understand the science behind it until you explained. The sticks are annoyingly expensive though. 1 stick = £1 I tried simple sticks instead but I gotta admit, I find them more stressful than CBFM.

    Good luck with the natural cycle!!!

    • Thanks!

      I agree that they are ridiculously expensive (though a bit cheaper online than at the drug store). In my experience, you can strategically skip sticks now and then and it still works. Obviously, you wouldn’t want to skip when it’s close to your peak day…but if you’re like me, you probably have a pretty good idea of when that will be ahead of time. Also, once you get your ‘peak’ reading, you don’t need to use any more sticks – it ALWAYS gives a second day of ‘peak’, followed by one day of ‘high’, so why waste the sticks?

  2. Hello, I found you through the ICLW list. I know I’m early but wanted to say Hi anyway. I’m enjoying your blog and I really hope you get lucky this cycle. Oh, how lovely it would be to be THAT couple who gets to cancel their IVF cycle because they are pregnant. My fertility specialist tells me that couple is 1 in 500, so it’s better odds than winning the lottery. I also trained in Chemistry, although I moved on to molecular immunology and I’m just starting an IVF cycle. I am enjoying the chemistry stuff a lot and am looking forward to following your journey.

    • Welcome! I just checked out your blog and I love the drawings. I’m excited to follow along with another scientist. 🙂

      Also, as an immunologist, please don’t hold it against me if I got the post about how HPTs work is totally off! (Feeling a bit self-conscious now!) Best of luck on your cycle!

  3. Wow this looks great! I think it would be especially helpful for me because I am terrible at charting my BBT. I always wake up to pee and realize I should have checked my temp. first so I am constantly skipping days… thanks for sharing!

    • Thanks for visiting! Yeah, the temperature is definitely a bit more tricky. I travel a lot and have sporadic wake up times which often messes me up…

  4. That mug. I’m not sure I’d want to drink my coffee or tea out of something that says “Hormones of the Menstrual Cycle,” but I still kind of love it.
    From reading your posts, I can imagine your IVF homework will have quite a rigorous syllabus! I’m interested to read about what you learn 🙂

    • Yeah…I keep wondering whether my students would think it was hilarious or disgusting. I’m sure they would appreciate knowing that I give myself as much homework as I give them. 😉

  5. stupidstork

     /  May 18, 2013

    1. Smoooooooooooch. Smooch.

    2. This may be the first time in a long time I’ve ever been genuinely excited that I don’t ovulate.


  6. My fertility monitor freaking broke. I’m still mad about it.

  7. Miralen

     /  August 8, 2013

    Knalani, here are some excerpts from the journal articles on how the CBFM (or at least what I believe is the earlier version of it, ie clear plan fertility monitor) works. Also added a link to a patent application that describes how the assay works for E3G metabolite detection.

    Hum. Reprod. (2001) 16 (8): 1619-1624.
    doi: 10.1093/humrep/16.8.1619
    The test sticks simultaneously detect LH and E3G in early morning urine. The LH assay is a classical sandwich assay and as the concentration of LH in the urine increases then the intensity of the line formed on the test stick increases. The E3G assay is a competition assay and as the concentration of E3G increases then the corresponding line intensity decreases. The monitor optically measures the intensity of the lines that form on the test sticks after sampling. The corresponding signal is measured in percentage transmission units (%T). As the concentration of LH increases the associated signal increases, as the concentration of E3G increases the associated signal decreases.

    Am. J. Epidemiol. (2009) 169 (1): 105-112.
    doi: 10.1093/aje/kwn287
    First published online: October 30, 2008
    The test stick has a nitrocellulose strip with an anti-LH antibody zone and an E3G conjugate zone. The monitor optically reads the level of E3G and LH in the urine by the intensity of the lines in the corresponding zones (13).

    E3G assay info here http://www.google.com/patents/WO1995013543A1?cl=en

  8. Erin

     /  July 10, 2015

    This is an awesome post, I’ve been freaking out (doing IUI and waiting for surge) I looked at my test strip this AM after I got only a “high” fertility again on day 14…..and was starting to doubt my monitor because I am extremely regular. Anyway, this was very useful and reassuring that my monitor knows what it is doing.

  9. Faith

     /  November 13, 2016

    I found your post and would like to ask you an question.
    First, how low does the clearblue monitor read the lh level? I had the eggwhite mucus last night and this morning the monitor still read low.

    • Hi Faith. My understanding is that the eggwhite mucous is the result of rising estrogen levels. When the estrogen level gets high enough, your monitor should read ‘High’. Eventually, the estrogen rises high enough that it triggers the LH surge, causing the monitor to read ‘Peak’. Being an infertile near menopause, I don’t know what is normal, but I often will have several days of eggwhite before the peak. Perhaps your estrogen levels are rising and causing the eggwhite even before causing a ‘high’ reading on the monitor? Best of luck to you!

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