OPK’s vs. BBT Charting

When am I Ovulating? A Discussion on OPK’s vs BBT Charting

Many women become confused when their OPK’s (Ovulation Predictor Kits) and BBT’s (Basal Body Temperatures) don’t agree on the day of ovulation- understandably so. This is one of the more confusing topics I’ve researched! We’ll file this under “advanced topics”. Here’s why.

Background: What are OPK’s and BBT’s anyway?

OPK: Ovulation Predictor Kits are tests that you can get at most any drugstore which have the purpose of predicting ovulation. The idea is that they detect a hormone called LH, or Leuteinizing Hormone in the urine. Once they read positive, usually it’s safe to say that you’ll ovulate within 12-36 hours.

BBT: Basal Body Temperature. I’ll just give a very brief summary of this one- basically by taking your temperature every morning, the second you wake up, you can tell whether or not you have ovulated. Before ovulation, the temperatures are generally 97.0-97.6F, and after ovulation the temperatures rise about 0.5 degrees, but should be somewhere around 98 degrees F. Why does the temperature rise? Because once the egg is released, the remaining sac (called the corpus luteum) releases progesterone for about 10-16 days. This is what causes your temperature to increase and is the reason why if you don’t get pregnant, most will get their period 10-16 days after ovulation. Keep in mind, the temperatures I’ve given are general ranges and may be very different from person to person. The important thing to realize here is that BBT charts are retrospective, meaning they don’t help in predicting ovulation, only in confirming (by 3 consecutive elevated temperatures) that ovulation has in fact occurred. For complete information and instructions on using this method, I recommend reading “Taking Charge of Your Fertility” by Toni Weschler.

Here are a few of the questions I’ve gotten from women who were confused about these two methods:

“My OPK was positive two days after my BBT spiked! What’s going on?!”

Hormonal levels are in a continuum- high or low, but almost always changing very gradually. The LH hormone (ovulation predictor) peaks 24-36 hours before ovulation, but keep in mind that it doesn’t immediately decrease- it can peak for several days. For some women, LH levels are still relatively high even after ovulation. And, depending on the brand of OPK you used, it might be able to detect relatively low levels of LH. The important thing for women trying to get pregnant to remember is that the first time you get a positive OPK, 12-36 hours later is normally when you will ovulate (see below for exceptions… of course there are exceptions!)

“My OPK was positive, but my temperature didn’t spike until 10 days later.”

This is because before the egg is actually released, you may have “mini-surges” of LH hormone which are picked up by an OPK as positive. The amount of LH in your body relative to the amount that is detectable by an OPK varies from woman to woman, and from one OPK manufacturer to the next. Always keep in mind, an OPK can predict ovulation, but doesn’t confirm that it definitely happened. Only your temperatures (or an ultrasound, or a pregnancy) can do that.

“My temperatures have spiked but I never had a positive OPK. Did I ovulate?”

Yes, most likely you did! This brings up the importance of using your OPK more than once per day. The majority of the time, LH is just beginning to rise in the early morning hours, so this is why testing first thing in the morning almost always gives a negative reading. If you think ovulation should be happening soon, keep testing, 3 times per day to make sure you don’t miss the LH surge. It is possible for it to last less than 24 hours, so for some it’s easy to miss! The other, more rare possibility in this scenario is that your body doesn’t make LH levels high enough for the type of OPK you are using to detect. So, while your body is still ovulating and still responding to the low LH levels, the OPK simply can’t detect it.

“Which method is better?”

Good question! My answer- I encourage everyone to try charting their BBT for a few months just to get the hang of it and gain a greater understanding of the patterns of your individual cycle. If your cycles are quite regular, then you really don’t need to buy the sometimes costly OPK’s every month- between knowing when you normally ovulate and other signs and symptoms such as cervical fluid monitoring, you should have a pretty good general idea of when the best time is to try to conceive. I recommend every other day beginning about 10 days before usual ovulation, continuing through until after ovulation is confirmed by 3 consecutive high temperatures.

However, if your cycles are fairly irregular and you are having a difficult time discerning other indicators of fertility (such as differences in cervical fluid and position), OPK’s can be very useful in helping you predict ovulation and correctly time intercourse.

This is just a quick introduction to give you an idea of why each method is used, and that there are some at-time confusing limitations to both methods and many exceptions not discussed here. While it can be reassuring to understand exactly what happened with every OPK test or temperature that you take, try not to get too wrapped up in the details. If there’s any question, try to conceive every other day throughout your cycle.