2022 01 25 opksandfemsense title

Why femSense and LH tests show different results - and that's OK

Ovulation tests are used to determine when you are most fertile and when is the best time to get pregnant. So how can it be that ovulation test strips and femSense sometimes show ovulation on different days - but you only ovulate on one day? It's actually quite logical when you understand how Ovulation tests work. The two different methods measure different body signals of hormonal origin that indicate ovulation. These signals occur in different phases of the fertile window. This begins with a hormonal surge at the start of the fertile window to allow the egg to mature in the follicle and ends around 24 hours after the egg is released during ovulation.

Ovulation test strips measure the LH (luteinising hormone) level in the urine and are therefore usually referred to colloquially as "LH tests". While they only indicate that the body is preparing for ovulation, femSense uses the Temperature methodto confirm that ovulation has actually taken place. In other words: LH tests measure the beginning of the fertile window, the temperature method measures the end.

But now to the functionality of the individual methods for determining the fruitful timeand what they say about your ovulation.

Ovulation tests

The use of ovulation tests helps you to determine your most fertile days in your cycle and indicates when you are most likely to conceive. There are several different methods for predicting or detecting ovulation: these include ovulation test strips (LH tests), the Billings or Cervical mucus method or the temperature method.

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Each system measures a different body signal that indicates that a woman is (probably) in her most fertile phase.

LH tests indicate approaching ovulation

LH tests are strips or sticks that you pee on to measure a specific hormone concentration. LH tests are strips or sticks that can measure the concentration of luteinising hormone in the urine. They are used in the days before They are used before ovulation and show you that your body is preparing for ovulation. Ovulation test strips measure the concentration of LH (luteinising hormone) in your urine. The LH level rises in the days before ovulation to trigger the release of an egg. This LH surge usually occurs 24 to 40 hours before ovulation. In short, ovulation test strips predict ovulation a day or two in advance - but give no guarantee that ovulation has actually taken place.

The temperature method recognises when ovulation has taken place

femSense uses the temperature method to measure and confirm ovulation. When a woman ovulates, her basal body temperature rises by a few tenths of a degree and remains slightly higher in the second half of the cycle. The higher progesterone levels in the second half of the cyclethat trigger this tiny rise in temperature are released by the corpus luteum, the remnant of the follicle that has released a mature egg during ovulation. This slight rise in body temperature is therefore a direct consequence and a reliable indication that ovulation has taken place. If none had taken place, there would also be no rise in temperature.

In short: the temperature method recognises ovulation to ovulation. And in fact, the temperature method is the only "home test" that can confirm ovulation.

Use LH tests and femSense together

So if you want to get pregnant, you shouldn't wait for the post-ovulatory temperature rise. Fortunately, there is competent help from certified medical products such as femSense! The femSense App can predict the start of your fertile window and the femSense sensor patch can then measure and confirm ovulation. And the algorithm learns! In short - femSense does all the hard work for you. So you no longer have to pee on dozens of sticks, take your temperature for minutes at dawn or analyse charts. The app clearly tells you when you are most fertile and most likely to get pregnant.

However, we know that many of our users who Use femSense to get pregnantwant to use all the tools available to them. Completely understandable! When you've made the decision to have a baby, it can't happen fast enough. In this situation, it's not a bad idea to use ovulation tests and femSense together to confirm your entire fertile window from start to finish. Whilst it's not absolutely necessary, the methods can complement each other. Just remember that they measure different phases of your fertile window, and don't be discouraged if the results aren't identical at first glance.

Women who with hormone-free contraception should know that LH tests are not a reliable contraceptive. Among other things, they are not always completely accurate or reliable, as their results can be influenced too easily. For example, the LH concentration in urine is higher in the morning than during the rest of the day, and is diluted when you drink more. Also, the rise in LH levels before ovulation varies from woman to woman, which is why the less sensitive tests sometimes fail to detect the rise - and the more sensitive tests can give a 'false positive'.

Scientists have also discovered that some women have several LH surges in one cycle. It is possible that the body "starts up" several times. However, as women usually only ovulate once per cycle, it cannot be linked to all these LH surges. An LH test would recognise these surges and incorrectly predict ovulation. You can find more detailed information here in our blog read more.

To create a better overview of the length of your cycleTo keep track of your fertile window and ovulation, you can simply track your cycle. Download the free femSense app to track your cycle, and use our sensor patches to know exactly when you are ovulating.


Galan (2023), LH Surge: Timing Ovulation for Fertility

Hsui-Wei et al (2023), Detection of ovulation, a review of currently available methods

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