How to Use Ovulation Predictor Kits

If you have been trying to conceive for a few months without success, ovulation predictor kits (OPK’s) may be helpful in identifying your “fertile window.” In my clinical practice, I recommend the use of OPK’s both as a tool for pinpointing ovulation, and for diagnosing potential ovulatory issues.

OPK’s measure the amount of luteinizing hormone (LH) in your urine – while LH is always present in urine, it increases 24-48 hours prior to ovulation. The LH surge is what triggers ovulation, the release of an egg from the ovaries.

The “fertile window” for most women is the day of ovulation, and the five days BEFORE. Once an egg is released, it only has a lifespan of ~24 hours to be fertilized before it disintegrates, BUT sperm are able to survive within a woman’s reproductive tract for up to five days after intercourse, therefore, intercourse within the five days preceding ovulation could also result in conception. (However, conception is more likely to occur with intercourse closer to the day of ovulation.)

How soon after a positive OPK will ovulation occur? For most women, ovulation will occur 12-36 hours after a positive test. Ideally, you will be able to interpret your OPK results in conjunction with other signs and symptoms of ovulation (e.g., cervical mucus, increase in libido, ovarian sensations, etc…) to get a better sense of when in that time period ovulation happens for you.

What is the best timing for intercourse? The day of your OPK positive, and the day after, is your best days for intercourse. (You may also consider the second day after a positive for “insurance.”) Because sperm may survive up to five days within a woman’s reproductive tract, intercourse in the days preceding your OPK positive, are also good times to try. If you are trying to conceive via intrauterine insemination (IUI), OPK’s may be helpful in calculating your best timing. (Most doctors will schedule the procedure ~36 hours after an LH surge is first detected.)

When should you start testing? If your cycles are fairly regular, begin testing 4-5 days before you expect to ovulate. For example, with a 27 day cycle, you can calculate your day to start testing by subtracting 14 (the length of a stable luteal phase) from the total length of your cycle; in this example, an OPK positive is likely around Day 13, therefore, you would begin testing on Day 8 or 9. The goal is to not miss your LH surge. With irregular cycles, use your shortest cycle length to estimate when to begin testing.

Chart of possible OPK test results.

Interpreting your OPK results: There are many OPK’s on the market. The brand that I recommend to my clients is Wondfo – they are inexpensive (Amazon), and in my experience, reliable and easy to use. There are two lines on a test strip – a control line and a test line. When interpreting an OPK, the test line should be as dark as, or darker than, the control line. Some women experience a gradual darkening of the test line as they enter their fertile window; other women experience a sudden darkening of the test line (which may be missed if you are only testing once per day, or using FMU). If using an OPK feels clumsy at first – after a cycle or two, you will likely feel more confident in interpreting your OPK results as you become more in tune with your body’s ovulation rhythm.

Example of one woman's LH progression, with positive strip at the top.

Have intercourse in the days preceding your expected positive – there are a number of reasons for this: you are within your fertile window and this will increase your chances of conception, as unfertilized eggs disintegrate ~24 hours post-ovulation; conception happens in the fallopian tubes, so ideally the sperm are there waiting for the egg to be released; increased sperm quality the day of your OPK positive (i.e., better motility), and the day after, your ideal days for intercourse. (*Please note, if there are male factor issues, an alternative intercourse schedule may be advised.)

While OPK’s work for most women, they may not be reliable for everyone. Some women naturally have lower LH levels, and OPK’s may not be calibrated best for your body; in this case, a woman may never get a true positive reading, but is indeed ovulating. In cases of polycystic ovarian syndrome (PCOS), LH levels tend to be high on other days of the cycle, not just preceding ovulation; in this case, a woman will get false positives.

It is important to note that a positive OPK does not confirm ovulation – an LH surge can happen without an egg being released. Stress, illness, perimenopause, certain medications, and some health conditions, may negatively impact ovulation. If there is any question as to whether you are ovulating, this can be investigated and confirmed through basal body temperature (BBT) charting, or a progesterone test during the luteal phase (a blood test ordered by your doctor).

If you use OPK’s consistently and do not get a positive result, some reasons for this include: a short surge that may have been missed; a long cycle – you started testing too early, and stopped testing before your LH surge; you truly did not ovulate (see reasons above); or, you unfortunately tested with a bad batch of sticks.

In my clinical practice, I strongly recommend against using popular “fertility app’s” to predict ovulation. Why? In my experience, many women who are trying to conceive using these app’s are missing their ideal days for intercourse (the day before ovulation, and the day of) – yes, the app’s become “smarter” over time as you input your cycle information, however, simply put, these programs use an algorithm to calculate ovulation, which can not take into account other human variables that may affect ovulation. While I do recommend using app’s to track your cycle, fertility app’s are NOT a substitute for OPK’s, and even more so, for being in touch with your body.

If you have been struggling to conceive, and would like to connect for a consultation, you may contact me at I use both traditional Chinese medicine and Western diagnostics in my evaluation of your case. I work with couples and individuals working to conceive both “naturally,” and through assisted reproductive methods, including IUI and IVF.