Basal body temperature charting can tell you if and when ovulation occurred. Confirming ovulation is helpful to rule out anovulatory cycles where ovulation does not occur. In hindsight, there is no chance of pregnancy during these cycles. Understanding when ovulation did occur can be helpful to review your chances for conception based upon recorded intercourse. It can also help some women see ovulation patterns from cycle to cycle.
Basal Body Temperature Charting
Basal body temperature (BBT) charting is a useful mechanism for verifying ovulation. It is not capable of predicting ovulation until after it already occurred. Charting of other symptoms like cervical mucus, LH testing, or using a fertility monitor can predict ovulation in advance. This is especially important if you are trying to get pregnant.
When ovulation occurs, the body temperature increases slightly (by about 0.5 degrees Fahrenheit or 0.3 degrees Celsius). This increase in temperature is sustained for several days. By monitoring and charting temperature, it is possible to determine ovulation by noting this temperature shift.
During the first part of the menstrual cycle, basal body temperature remains relatively low. This lasts throughout the follicular phase. Once ovulation occurs, progesterone levels in the body increase. The higher progesterone causes the body's core temperature to shift by fractions of a degree. This temperature remains elevated for most of the luteal phase until progesterone levels drop. If pregnancy results, progesterone will remain high and the body temperature will remain elevated.
An ideal temperature chart shows two different temperature levels. This type of chart is called biphasic. This is because it depicts the two temperature phases of the menstrual cycle. A lower temperature level exists during the follicular phase. The luteal phase sees an increased temperature after ovulation that is sustained until the start of the next cycle.
The following describes the process of taking your temperature.
- Use a thermometer that is accurate to at least 0.1 degrees.
- Temperature can be taken either orally or vaginally as long as the same method is used for consistency.
- Take your temperature immediately after waking each morning before doing anything. Your thermometer should be kept bedside so you can take it without getting out of bed.
- Record the temperature on a paper chart or using software. Connect individual temperature readings with a line if drawing charts manually.
- If any special circumstances exist that may affect your temperature, note them with your data (e.g. illness, late waking, trouble sleeping, etc.) and circle the temperature point to indicate uncertainty.
Once several temperatures are charted, the chart can be analyzed to detect for ovulation. The following procedure explains how this analysis is performed.
- Regularly check your temperature chart in search of an upward and sustained shift in temperature.
- If your temperature shifts up noticeably and remains elevated, you can confirm ovulation using the three over six rule. This requires that the 3 temperatures after ovulation be 0.2 degrees F (0.1 degrees C) or more above the highest of the previous six temperatures with at least one of the temperatures being at least 0.4 degrees higher. This scenario is demonstrated in the adjacent figure.
- After a temperature shift, you can optionally draw a coverline for reference. This line reflects the highest temperature of the previous six being considered plus 0.2 degrees. You want to see the prior six temperatures at least 0.2 degrees below the line and the next three above it to confirm ovulation.
- Once this criteria is met, ovulation can be confirmed. Ovulation coincides with the temperature shift.
Charting Temperature Data
With charting software all you need to do is enter your daily temperatures. The software will take care of plotting and even interpreting the temperatures. When charting on paper you must plot and connect the lines manually. Shade or circle the box that corresponds to your temperature reading. You can then draw lines to connect the individual temperature readings.
When working with a paper chart, the fractional portion of the temperature is indicated in the individual numbered boxes. The whole part of the reading is written in the column. For example, a temperature of 98.6 would circle square 6 with 98 written in the margin to the left.
Around the middle of your cycle, watch carefully for an upward shift in your temperatures. Once you see 3 or more temperatures that are 0.2 degrees F or higher than the previous six temperatures, ovulation can be confirmed. You do not need to wait for a 4th temperature to be above the line if one of the temperatures is at least 0.2 degrees higher than the coverline. You can use the three over six rule described previously to help make this determination. Coverline techniques can alternatively be used to interpret your BBT chart. For more information and options, see the analysis page.
Benefits of BBT Charting
Couples trying to conceive can benefit from temperature charting by seeing patterns in ovulation from cycle to cycle. This is only feasible to the extent that your cycles are regular. Regardless of regularity, confirming ovulation is of medical significance to know that pregnancy was possible. It is also helpful to reflect on the cycle and gauge the likelihood of conception based upon when couples had sex.
Couples interested in avoiding pregnancy can benefit from temperature charting by knowing when ovulation has passed. Once ovulation has passed the likelihood of conception falls dramatically. Unless ovulation was incorrectly identified or a second ovulation takes place a day or two later (what occurs in the case of paternal twins), the chance of pregnancy falls to zero within a 24 hours or so of ovulation. With pregnancy unlikely, intercourse can safely resume. Since there can be some error in estimating ovulation, it's generally recommended not to assume that fertility has subsided until a few days after the ovulation estimate.
- Proven to be one of the most effective symptoms. Very commonly used by fertility awareness practitioners.
- Quantitative data is easy to record.
- Useful to identify when ovulation has passed and the likelihood of conception declines.
- Offers a double-check against other fertility indicators. This is useful to reflect back over a given cycle and consider the likelihood of achieving pregnancy based upon when intercourse was recorded relative to ovulation.
- Can be used to estimate luteal phase length across many cycles.
- Does not identify ovulation until after it has already passed.
- Requires consistency in taking daily temperature readings to be effective.
- Environmental and other factors can influence temperature and make the temperature shift due to hormonal changes harder to detect.
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