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Home > Technical questions about OvuSense Sensor and App > My doctor says I ovulated on a different day to what OvuSense says. Why is that?

My doctor says I ovulated on a different day to what OvuSense says. Why is that?

Usually, your doctor will carry out one of two tests to determine if and when you ovulated.

The most common test is serum progesterone, often called “21-day progesterone”. The value of serum testing has long been established, with a “serum concentration of greater than 3 ng/ml” providing “presumptive but reliable evidence of recent ovulation” ASRM (2012) referencing Wathen et al. (1984) – study size 79. In Europe, the cut-off is generally expressed in nmol/l where >30nmol/l is indicative of ovulation. However, blood tests are inconvenient for the clinic and patient, and will always suffer from issues of timing of the blood draw even in a patient with a regular cycle – with false negatives proving a common issue. The ASRM (American Society Reproductive Medicine) guidelines go on to state that “a serum progesterone measurement generally should be obtained approximately one week before the expected onset of the next menses, rather than on any one specific cycle day (e.g. cycle day 21).”

This test cannot predict when you are about to ovulate in a cycle at all, and cannot provide a particularly reliable date of ovulation after the event either, as progesterone levels vary quite widely between women and from cycle to cycle, meaning that a positive result can occur over a 2-3 span. If the test is positive, there is a generally held view that ovulation occurs 7 days before the test is carried out which means day 14 in a woman with a 28-day cycle, but this is a much less reliable indicator than OvuSense as a serum progesterone result is only a spot test. It is also important to know that a negative serum progesterone result does not necessarily mean no ovulation occurred in that cycle.

Here is a testimonial from an OvuSense user that shows how it can help in practical terms: “I just thought you may appreciate some positive feedback. After I used Ovusense, I rebooked my ‘21 day’ Progesterone blood test for the week later and the score has now come back as 50…. I am so pleased I invested in Ovusense because for 5 years I have presumed I have ovulated 10 days earlier than I actually do and have struggled to get pregnant. Thank you.” – Alison – October 2014.

The alternative test is an ultrasound folliculometry scan. Three to four scans per cycle are the suggested requirement of good cycle monitoring. However, because a scan is a moment-in-time 'spot test', timing is everything. Your clinician will usually be looking for a dominant follicle (the next one that is most likely to rupture resulting in ovulation) in one ovary of 20mm or more in size, meaning that you are likely to ovulate in the next 24-48 hours. They may also look for evidence that a follicle has recently ruptured, meaning you have already ovulated during that cycle. However, folliculometry is not always a good predictor of ovulation and is not surprising given that it holds that all women of whatever size and shape, or hormonal makeup will have identically sized follicles just before follicular rupture. Dominant follicle size can vary greatly as shown by: Vlaisavljević and Došen (2007). It is therefore important to understand the result your clinician provides you in the context of your whole cycle. The best approach rather than estimating when ovulation will occur for you and scheduling based on this best guess, you can use the results from OvuSense to schedule ultrasound scans with your doctor which will provide you with much more detailed information about that cycle.