Did you know that you are fertile on up to six days of your monthly cycle: ovulation day, and the five days leading up to ovulation. This takes the possible length of sperm survival into account, as sperm can survive inside your body for up to five days.
The main factor the Natural Cycles algorithm takes into account is your temperature. During your menstrual cycle your temperature rises and falls slightly which is due to the change in progesterone hormone levels. When we talk about temperature we refer to basal body temperature, which is the lowest body temperature when at rest, and can be measured with a two decimal basal thermometer immediately after waking and before getting up and out of bed.
Ovulation is when an egg is released from the ovaries. This is how the body prepares for the arrival of sperm and possible fertilization.
The menstrual cycle consists of two phases: the pre-ovulatory (follicular) phase and the post-ovulatory (luteal) phase, which are separated by ovulation. Your cycle begins on the first day of your period (CD1) and lasts 29 days on average. The day of ovulation and cycle length can vary significantly from woman to woman, as well as from one cycle to the next.
In the first phase, the follicular phase, your body temperature is a little lower and progesterone causes your temperature to rise in the second luteal phase, once ovulation has occurred. Your body temperature is known to rise by 0.36 – 0.81F due to the increase of progesterone hormone levels. This is the main indicator the Natural Cycles algorithm takes into account when calculating your daily status and predictions. Another optional indicator is the Luteinizing Hormones (LH) which surges just before ovulation occurs.
There are certain indicators in your body that signal what stage of your cycle you are in. Natural Cycles uses the main indicator, which is basal body temperature, as well as your period dates. You can also input results from urine tests that identify the surge in Luteinizing Hormone (LH), which occurs just before ovulation. These are the most scientifically accurate and objective measurements.