What you can expect

To use the basal body temperature method:

• Take your basal body temperature every morning before getting out of bed. Use a digital oral thermometer or one specifically designed to measure basal body temperature. Make sure you get at least three hours of uninterrupted sleep each night to ensure an accurate reading.

  For the most accurate results, always take your temperature using the same method. Try to take your temperature at the same time each day, when you first wake up.

• Track your temperature readings. Record your daily basal body temperature and look for a pattern to emerge. You can do this on a paper chart or an app designed for this purpose.

  Basal body temperature may increase slightly — typically less than a 1/2 degree F (0.3 C) — when you ovulate. Ovulation has likely occurred when the slightly higher temperature remains steady for three days or more.

• Plan sex carefully during fertile days. You're most fertile about two days before your basal body temperature rises, but sperm can live up to five days in your reproductive tract.

  If you're hoping to get pregnant, this is the time to have sex. If you're hoping to avoid pregnancy, unprotected sex is off-limits from the start of your menstrual period until three to four days after your basal body temperature rises — every month.

Although there are numerous apps available for tracking menstrual cycles, only one is approved by the U.S. Food and Drug Administration (FDA) for pregnancy prevention.

Natural Cycles uses an algorithm to calculate the days during your cycle when you're more likely to be fertile. The app calculates your fertile days based on daily temperature readings as well as other information you input about your menstrual cycle.