Interpreting temperature

Of the three fertility signs - cervical fluid, cervix changes, and basal temperature - temperature is the most objective and easily observed. Most couples who use NFP prefer to use a basal thermometer, which is designed to be give more accurate readings. Typically household thermometers are designed for detecting fevers, but if you do not want to purchase a basal thermometer, a decent digital thermometer will work fine. If you are interested in purchasing a basal thermometer, there are very affordable options online. This is the thermometer I use. I haven't had any luck finding basal thermometers in local drug stores, so if you know of a store that regularly carries them, let me know in the comments!

There are many things that will effect your temperature - exercise, time of day, the temperature of the surroundings, alcohol, illness, etc. We want to control for as many of these factors as possible, so the temperature differences we observe are caused by changes in the concentration of the hormone progesterone. The easiest way to control for these factors is to record your temperature at the same time each morning, before you get out of bed. I am not a morning person (understatement!) so I set the alarm on my phone for 7:00 am every morning, the earliest time I get up during the week. I keep the thermometer and a pad of paper on the nightstand and, on mornings that I can sleep in, I wake up just enough to roll over and take my temperature.




Of course, some factors are hard to control, like getting sick. If you suspect that an outside variable has affected your temperature, make a note in your records. If it's only a day, it will still be easy to see patterns. If it's several days it may be harder, but luckily there are other symptoms to look for.

Here's a typical chart of temperatures:

There's a dramatic temperature shift between day 16 and day 17. The blue line is drawn 0.4 degrees higher than the highest temperature observed for the seven days prior to the shift. After 3 temperatures are observed above the blue line, we can be fairly confident that the temperature shift indicates that ovulation has occurred. Ovulation typically occurs about a day before the temperature shift, so ovulation probably occurred on day 15 or 16.

Here's a few things you could learn from temperatures:

• Low temperatures - Temperatures that are consistently low (under ---) can be a symptom of hypothyroidism. If you notice that you have consistently low temperatures, it's a good idea to bring this up with your doctor.
• Illness - Is your temperature higher than it normally is? Do you feel not quite yourself? It's possible you're sick, and the increased temperature is your body fighting it off.