Find Your Most Fertile Days: A Guide to Tracking Your Fertility

A woman who charts her fertility signs and has well-timed intercourse with her partner is encouraged by NFP teachers to consult her doctor after three cycles with no success -- compare that with the 12 unsuccessful cycles that doctors suggest waiting when a couple tries to conceive simply by stopping birth control.

NFP/FAM is also cheaper than ovulation predictor kits. The kits will cost the average couple between $15 and $60 per month, but a $10 thermometer is all that is needed to use fertility signs to predict ovulation.

A woman's fertility varies throughout her cycle. She is most likely to become pregnant if she and her partner have sex in the few days leading up to and on the day of ovulation. This fertile window is about three to four days for the average woman. When a woman uses NFP/FAM to achieve pregnancy, her goal is to use her fertility sign information to accurately pinpoint her fertility window and to have sex during that time.

There are two main fertility signs. The first sign is a woman's body temperature upon waking each morning, called the basal body temperature, or BBT. This temperature helps indicate levels of the hormone progesterone.

In a normal cycle, a woman's progesterone level will start out low during her period and remain low until ovulation. After ovulation, the progesterone level rises and will then stay high until her period begins again. Because one of progesterone's main side effects is to raise a woman's BBT, the high level of progesterone after ovulation reveals itself in a high daily BBT reading.

When plotted on a chart each day, a woman will see that before ovulation her body temperature stays in a low range, while after ovulation it is in a higher range. Generally the BBT will remain in the post-ovulatory high range for a total of 12 to 16 days (often called the luteal phase), and will then fall back to a low range as the woman's period begins.

The temperature sign can quickly reveal that something is wrong. If a whole cycle goes by -- period to period -- without any movement of the BBT to a high range, a woman can tell that she did not ovulate that cycle. While it's common to have a cycle without ovulation every now and then, perhaps once per year, seeing cycle after cycle like this indicates that there is a problem that needs a doctor's attention.

Melissa Gentile, of Reading, Massachusetts, discovered another type of problem through her BBT. "I realized I had a slightly short luteal phase -- only 10 days," she says. Short luteal phases can sometimes mean that a woman's hormonal balance is off, hampering her ability to become pregnant. After using progesterone supplements prescribed by her doctor, Gentile was able to successfully conceive her daughter after two cycles.