Are You Using LH Strips Wrong?

(It could be the reason you’re still not pregnant…)

If you’ve been trying to conceive for any length of time (or at least thinking about it for any length of time) you have probably heard of LH strips (AKA OPKs, LH test, ovulation test, luteinizing hormone tests, ovulation predictor kits). LH strips are a pretty standard tool in the TTC communities and are often recommended to help improve odds of getting pregnant in any given cycle. But the way you use LH strips, whether correctly or incorrectly, could make or break your TTC efforts. This article will help you avoid making this common mistake.

What are LH strips?

LH strips are home urine tests that can be used to identify the surge of luteinizing hormone (LH). LH strips, when used correctly, predict ovulation to help couples who are trying to conceive by improving the precision of their efforts. Conception really only is possible during a six-ish day window of the menstrual cycle, corresponding to the five days leading up to and including ovulation, plus the day after. [Learn more about fertility, hormones, and the fertile window in my book, Go with Your Flow.] Contrary to most Sex Ed classes (or what is exemplified in the movie Mean Girls) a single sexual encounter is actually less likely to result in pregnancy then it is to cause it, statistically speaking. 

Knowing this, most cycle tracking apps offer a predicted fertile window to aid users in identifying the days of the month when they are likely to conceive from intercourse. Now, of course, these apps make their predictions based on averages (which ironically do not actually reflect the average woman) that assume a luteal phase length of 14 days, with ovulation occurring approximately mid-cycle.

To give you a few examples:

• For a woman with an average cycle length of 28 days, her app would predict ovulation on day 14, with a 14-day luteal phase thereafter
• A woman with a 35 day cycle would receive a prediction for ovulation on cycle day 21 (again with a 14-day luteal phase)
• A woman with a 24 day cycle would receive a prediction for ovulation on cycle day 10 (based on a 14-day assumed luteal phase)

While any of these scenarios could be true for a given woman, it is also true (and especially important to note) that countless other variations are also possible.

Why?

Because not all luteal phases are 14 days in length.

Let’s go back to the “textbook” 28-day cycle for a moment: this woman may have:

• A 14-day luteal phase with ovulation on day 14
• But she just as easily may have a 12-day luteal phase with ovulation on day 16.
• Or even the somewhat atypical 15 (or more) day luteal phase with ovulation on day 13.
• She also could have a less-optimal but still potentially fertile 8-day luteal phase with ovulation on day 20. 

Even within just this 7 day variation, you can see easily that miscalculating the day of ovulation could mean completely missing the entire fertile window. Yikes.

So, instead of relying on the law of averages and missing the important detail of individual biology, scientists have come up with easy, at-home ways for tracking ovulation, based on the hormonal dance that takes place throughout a menstrual cycle. Ergo, LH strips or “ovulation predictor kits.”

How Do LH Strips Work?

In order to understand the lowly LH strip, we first need to understand it’s namesake hormone, LH–or luteinizing hormone.

In a nutshell, luteinizing hormone is a chemical signal sent by the brain to tell the ovary to release an egg.

The menstrual cycle is divided into two parts (which are, as we already learned, not necessarily halves) by ovulation, marking the transition point between the two.

The first part of the cycle, called the follicular phase is all about the development of an ovarian follicle (AKA egg) which bursts open during the event of ovulation. If a sperm is there, ready and waiting, when this occurs, fertilization and subsequent conception could theoretically occur. (Note: It goes without saying that many other factors are involved when determining whether or not pregnancy occurs, but those are beyond the scope of this article.) 

An Overview of Ovulation

During the follicular phase, the brain releases FSH, or follicle stimulating hormone, to facilitate maturation of the egg. As the egg matures, it produces increasing levels of estrogen, which the brain monitors. In addition to thickening and developing the uterine lining to prepare for a possible pregnancy (or period), this rising estrogen level serves as a communication signal to the brain to keep the nervous system in the loop about the status of the egg and it’s development. Once estrogen levels reach a high enough point, the brain then responds with its own signal (LH) to trigger ovulation.

After the follicle releases its egg, it then transforms into a progesterone-producing gland called the corpus luteum, which serves as the namesake for the second part of the cycle: the luteal phase. 

In reality, LH is produced at least a little bit every day of the cycle, even when menstruating. However, the key differentiating factor between “everyday LH” and the LH surge is the tremendous increase in amount of LH being released in any given moment. For younger women, the daily average LH level may be less than 1.0 mIU/mL, though for someone who is perimenopausal, it may fall into the tens-range. With regards to the brain-ovary line of communication, it’s not so much the actual numerical value of LH as much as it is the magnitude of change. To trigger ovulation typically LH levels must rise by a factor of 10, but for some people it is much more than this. It is not uncommon for LH levels to exceed 100 mIU/mL at the peak of the surge.

LH levels may also be high at a baseline in women with PMOS (formerly named PCOS) because of dysregulated estrogen signaling from polycystic ovaries. Both in the case of PCOS/PMOS and perimenopause, these slightly higher baseline LH levels may result in the rupture of more than one follicle at a time leading to a multiples pregnancy. This is why fraternal twins are statistically more common in women of older age and in women who have PCOS/PMOS. 

The Confusion About LH Strip Timing

Because LH starts to rise shortly before the release of the egg, and because conception is most likely if sperm are already ready and waiting in the fallopian tube at the time of ovulation, studies have identified that the most fertile days-–or the days of the cycle on which conception is most likely–-do not correspond to the day of ovulation itself, but rather to the day prior, and even the day before that. This allows sperm to make their way through the reproductive tract, undergo the capacitation process (which basically is just a fancy term for saying the sperm reaches its own peak maturity, which takes about 10 hours), and be ready to greet the egg when she make her arrival. 

To use an LH strip, a woman simply collects a urine sample, dips the test strip in the urine, and then waits a few minutes during which the LH present in her urine reacts with dye and adheres to a small line of antibody printed on the strip. Like a pregnancy test, LH strips also have a control line which is always dark in color. However, the key difference between a pregnancy test and an LH strip is the fact that LH is always present (whereas the pregnancy hormone hCG is not.) So, with an LH strip, a pale pink test line indicates a negative whereas a dark pink test line (which is as dark as, or darker than the control line) indicates a positive. 

But here’s the catch… LH surges for most women last multiple days. (Studies show an average of 4.8 duration.) Typically over this timeframe, the LH levels rise rapidly, peak, and then fall back down to baseline. While the actual length of the LH surge varies from woman to woman, the key factor here when predicting ovulation is to pay attention to the beginning of the LH surge rather than the peak. It’s the dramatic rising nature of LH that triggers ovulation, not the absolute maximum (as we have already discussed that the peak itself can vary in magnitude from woman to woman, as well as throughout a woman’s life, depending on the season of her fertility).

To flush this out a little bit more:

• One woman might have a 4-day LH surge with the peak occurring at the 48-hour mark
• Another woman might have a 1-day LH surge with the peak occurring at the 12 hour mark

Although the day of the peak itself varies in this scenario by over a day and a half, the important thing is that the beginning of the surge falls on the same day for both women. Because ovulation occurs roughly 12 to 36 hours after the start of the surge, both of these women would have ovulation happening on the same day. But for the first woman, her LH surge didn’t peak until after she actually ovulated. 

The real kicker is that this, statistically speaking, is the norm:

Medical researchers have found that for roughly 94% of ovulating women, LH peaked 1.2 days after ovulation already occurred. (Study link)

This means that if you are relying on the peak day, meaning the day that your LH strip is the darkest rather than the first day that you get a positive, and you are timing, your TTC efforts according to this, there is a very good chance that you will not be having sex until after ovulation has come and gone. Which means you aren’t going to get pregnant even if you have a perfectly healthy egg and a perfectly healthy sperm. (Chances of conception on the day after ovulation are 5% or less.)

Now this is an area of confusion that I often see among patients, in my real life conversations, and online. While many LH strips delineate the first positive day as being of highest importance in their instructions, the whole thing gets a little bit confusing because the test strips don’t provide numerical values; they simply use an analog, observation-based indicator. For women who are new to using LH strips, the color gradient can be kind of confusing. 

But let’s say you are among the women who know to count from the day of the first positive test. Well, because of the whole individual variation principle, and because of the variation among test sensitivity on LH strips from different manufacturers, you still might accidentally be missing the start of your LH surge. Here’s why…

How LH Strips Work 

I touched on this a little bit earlier, but to go into a bit more detail:

In an LH strip assay, urine flows across the strip and carries LH molecules toward antibodies attached to colored particles. The LH binds to these color-labeled antibodies, forming pink LH–antibody complexes. These complexes continue moving until they reach the test line, where another set of antibodies captures them. The accumulated colored particles produce the visible test line, with more LH generally producing a darker line. A separate control line confirms that the sample flowed correctly and the assay functioned properly.

Just like pregnancy test, which come indifferent sensitivity levels (i.e. 6.3 mIU/mL for the First Response Early Result compared to 25 mIU/mL for most standard tests and “internet cheapies”) LH strips can come in different sensitivity levels, generally ranging between 10 and 40 mIU/mL. An LH strip with a lower detection limit. (i.e. 10 mIU/mL) can pick up on lower level levels of LH hormone in the urine and will provide a brighter or darker test line. Meanwhile, an LH strip with a lower sensitivity. (i.e. 40 mIU/mL) will pick up LH in urine only when it has a higher level. Therefore, the same concentration of LH in the same urine sample would show up differently on these two tests: a darker line on the higher sensitivity LH strip (10 mIU/mL) and a lighter line on the lower sensitivity LH strip (40 mIU/mL).

If you are somebody who has a generally lower LH level and a comparatively lower peak, LH strips are going to show up lighter overall than someone who has a generally higher LH level. If you are using a high sensitivity test, this shouldn’t be a problem as it’s going to show test lines of the appropriate darkness and identify the beginning of your surge without issue. Like anyone else, your line will get darker as you reach the peak–but as we have already covered, it is the beginning of the surge that matters the most. This means that for the high sensitivity (10 mIU/mL) LH strip, the test line is equivalent in shade to the control line. But sometime thereafter, the test line would get darker than the control line (around your peak). However, if you were to use a lower sensitivity LH strip (i.e. 40 mIU/mL), your test line might not be dark enough at the start of your surge to indicate a positive test (as identified by a test line of equivalent darkness to the control line) even though, biologically speaking, the LH surge has started. 

In other words, you might not see a positive until you reach the peak of your LH surge which–if you are like the 94% of women in the world, is 1.2 days after ovulation rather than 12 to 36 hours before.

Understanding LH Strips in Real Life

Let’s take a look at this chart made using an Inito fertility monitor, which provides numerical data for all four fertility hormones (LH, estrogen, FSH and PdG, which represents progesterone).

As you can see on this chart, the purple line (LH) has risen dramatically from baseline levels. (This isn’t evident in the photo, but the day prior, the LH level was 0.08 mIU/mL). Having risen to 3.14 mIU/mL, it increased by several orders of magnitude, indicating the beginning of the LH surge. This indicates that ovulation will occur within 12-36 hours.

Now, let’s compare that same test to a standard, analog, over-the-counter LH strip:

The LH strip at the top of this photo was taken on the same day, with the same urine sample as the Inito reading. According to the definition of “positive” being the test line as dark as the control, this test would not be considered positive.

But we know that it is, because of the several days of preceding data identified with the Inito monitor.

Now let’s move on to the next day in this cycle:

On this next day, the Inito fertility monitor identified an LH level of 19.37 mIU/mL (corresponding to the peak of the surge) and the standard, over-the-counter LH strip turned positive (as evidenced by the bottom strip in the photo on the right, where you can also see the old, dried out strip from the day prior for comparison). The Inito identified the start of the LH surge on cycle day 12, whereas the OTC LH strip was negative. In reality, ovulation occurred sometime thereafter, and was likely complete by the time the LH surge reached its peak on day 13. The OTC LH strip did not identify a positive until cycle day 13, but as we know from Inito, ovulation had already likely occurred.

Here’s another example with Inito. In this cycle, the LH surge was identified on cycle day 16, with both the peak and the first positive taking place on the same day. In this case, the standard OTC test strip would be positive too.

Do you see the problem?

In short, standard, over-the-counter LH strips are great if you’re someone with a generally high LH level, but they may miss the start of the surge for women with lower levels (or more dilute urine). Missing the start of the surge by just one day could mean missing the day of ovulation altogether.

Of course, the obvious way around this is to make sure that you’re having intercourse (or, as it’s more commonly referred to on the online forums, “baby dancing”) daily throughout your fertile window. (If you don’t know when that is, I would suggest starting as soon as your period ends and continuing until you’re sure you’ve ovulated, whether via monitoring cervical mucus, basal body temperature, or with a device like Inito.)

Alternatively, you could check the sensitivity of your LH strips.

LH strips come with different sensitivity levels. When in doubt, use a high-sensitivity strip. Here are a few options:

• The Inito Fertility Monitor, which provides numerical data up to two tenths of a decimal (0.01 mIU/mL) for LH, plus the other 3 fertility hormones
• PreMom Quantitative Ovulation Predictor Kit and LH Strips which range from 0-65 mIU/mL
• Clearblue Advanced Digital Ovulation Test, which is sensitive to 40 mIU/mL
• Easy @ Home LH Strips, which observe 25-30 mIU/mL

The Bottom Line on LH Strips

If you’re using LH strips to time conception, don’t wait for the darkest test line to start trying. Pay attention to your month-to-month averages, and make sure you’re TTC well before your LH strips show a positive. View the LH positive as a signal for when you can stop trying (a few days after your positive) rather than a signal to start. Because a standard LH strip may miss the earliest part of your surge, consider testing more frequently as you approach your fertile window, and choose a more sensitive or quantitative test if your strips tend to turn positive late.

Most importantly, don’t put all your timing eggs in one hormonal basket: combine LH testing with cervical mucus, cycle patterns, or another method of confirming ovulation. The goal isn’t to perfectly predict the moment the egg is released, but to make sure sperm are already there waiting when it happens.