It is pretty simple to prepare a basal body temperature chart (BBT chart). All you need to do is note down your body temperature the first thing in the morning, using an appropriate BBT thermometer that records minute temperature variations.
The BBT chart you use is biphasic, just like your menstrual cycle. A coverline allows you to note low temperatures before ovulation in the follicular phase and high temperatures in the latter luteal phase after ovulation. A BBT chart allows you to know the time of ovulation, greatly increasing chances of pregnancy when you are planning to start a family, and works as a natural family-planning chart.
You should record your temperature at the same time of the day. Basal body temperature varies with time but it is okay if the average time stays within thirty minutes either side. However, it is crucial that you take your temperature before any physical activity, and that includes kissing your partner. The time should preferably be every morning on awakening or immediately after you have had at least three hours of sleep.
In the basal body temperature method you look for a temperature variation of at least 0.4 degrees Fahrenheit. Basal body temperature drops when you ovulate due to a surge in estrogen release and then suddenly shoots up when progesterone is released for preparing the uterus for pregnancy. Using the basal body temperature method involves studying the temperature variations and drawing a coverline – a line drawn just above the six highest readings preceding a spike in a basal body temperature chart, to segregate the follicular and luteal phase.
The important thing while drawing a coverline between your follicular and luteal phase is to ignore the high temperature that lasts for only one day. The shift in basal body temperature should be for at least three days or till the end of your menstrual cycle. Ovulation is suggested when the temperature rises and stays above the coverline for 7-8 days. It may take four to five cycles for you to fully understand a basal body temperature chart for proper estimation of time of ovulation.
To be doubly sure, use the basal body temperature method in conjunction with the changes that occur in the consistency of cervical mucus.