The Sympto-Thermal method of NFP works by interpreting two or three signs of fertility to indicate what phase the woman is in. For an overview of the female fertility cycle and what hormones are at play, please see The Science of NFP: Understanding the Hormones.
The first sign a woman learns to chart is her basal body temperature (BBT). A woman’s BBT helps her identify when she has ovulated. It must be taken at the same time every morning for accurate results. After ovulation, BBT rises considerably due to the corpus luteam producing progesterone. A woman’s BBT will stay in the high range (also known as the Luteal Phase) for approximately 10 to 14 days. Women with irregular periods, take heart! Typically, the length of time of the luteal phase is regular, whether or not your cycle length is always regular. It works like this: Woman A has cycles of 32, 45, and 37 days. That’s a big variance, but her luteal phase is always 12 days. Woman B might have perfectly clockwork cycles of 28 days. Her luteal phase is always 14 days. These women are mythical, but they show the rule. The overall length of your cycle does not indicate how long your luteal phase is. Each woman is different and so actual luteal phase lengths vary from woman to woman, with one woman having a 10 day luteal phase, another 13 days, and another 12 days. Once a woman determines how long her luteal phase typically is and can identify her ovulation, she will be able to accurately anticipate the date of her expected period, within 1 to 2 days.
Basal Body Temperature only helps the couple figure out if ovulation has already occurred. For this reason, STM also teaches the couple to observe the woman’s mucus pattern. As a woman’s body progresses through her fertility cycle, her body produces three basic types of mucus: not-fertile (the absence of mucus), slightly fertile (sticky/gummy mucus) and very fertile (eggwhite mucus). A man’s sperm have a lifetime of 20 minutes to 5 days, depending on what type of mucus the woman is producing and how healthy his swimmers are. A woman’s mucus provides a healthy environment, helps the sperm to navigate the cervical canal, and provides nutrients to sustain them in their journey. Because it is the mucus that will help the sperm meet the egg, it is important to learn to differentiate between the three basic types. When a woman has no mucus before her temperature shift, she can consider herself less likely to conceive. Once a woman begins to produce mucus, she considers herself more likely to conceive. When a couple is just starting to learn NFP, there are several rules which the instructor will clarify that the couple will need to observe while abstaining from sex. This is both to help a woman avoid becoming confused by seminal fluid and also so that couples who are trying to avoid or postpone pregnancy can be more assured that they have observed the correct rules for their situation. As the woman’s body gets closer to ovulation, her mucus will become more fertile and after ovulation her mucus will dry up thereby becoming once more
Once BBT and mucus are observed together, a more complete picture of the woman’s fertility takes shape. Mucus will help the couple determine when the woman is leaving her first infertile phase and entering her fertile phase, BBT and mucus will confirm ovulation, as well as determine when a woman is entering her second infertile phase, and BBT will indicate the length of a woman’s luteal phase.