During your menstrual cycle, your body goes through two phases separated by your ovulation day. These two phases are determined by Natural Cycles by you entering your basal body temperature into the app.
Natural Cycles makes a prediction for the upcoming ovulation day based on your historical data, such as average ovulation day, cycle length, etc. Every cycle is unique, therefore Natural Cycles does not assume that your ovulation will occur on exactly the same day as previous cycles but will calculate the most possible day. When the temperature has risen enough to confirm ovulation, the actual ovulation day gets calculated from the temperature curve.
If you are past the predicted ovulation day, you will not see the ovulation symbol until ovulation has been confirmed by the algorithm. Using LH tests when the app urges you to will help the algorithm to confirm ovulation faster.
The phases and their temperatures
During the first part of your cycle, the follicular phase, your body temperature is lower due to the high estrogen levels. This is also where your fertile window lies, the six days per cycle where intercourse could lead to a pregnancy. The follicular phase ends when ovulation occurs, which is on cycle day 17 on average for all our users.
Ovulation is when your ovary releases an egg, this egg travels through your fallopian tubes where it can be fertilized by sperm waiting there. The algorithm takes into consideration that sperm can survive up to 5 days in the female body. When ovulation has occurred the body enters the luteal phase and releases the progesterone, this hormone aids the fetus’ development if conception occurs. If conception did not occur the egg will be flushed out with the period later on.
During the luteal phase, the temperature is increased, which is easily seen on the graph the app will draw for you. Progesterone leads to an increased basal body temperature, on average an increase of 0.3 °C. This is a pretty big increase when you measure your basal body temperature, as the differences are so small. This is also why a basal thermometer must be used and not a regular fever thermometer!
The algorithm will confirm that you are no longer fertile, and a green color will be shown. The luteal phase is often very consistent, meaning your menstruation will most likely occur after the same amount of days in each cycle. This is valuable information that the algorithm is also using to determine correct ovulation day if the temperatures are fluctuating. The luteal phase ends as menstruation shows up, which starts a new cycle.
Here we can clearly see the temperature increase that occurs after ovulation, which is the increase that the algorithm is searching for in order to confirm that ovulation has occurred. Normally the algorithm require 2-3 high values before ovulation can be confirmed, and during those days the egg symbol may disappear – just keep measuring and the algorithm will soon be able to place your ovulation!