The basal body temperature method — a fertility awareness-based method — is a type of natural family planning. Your basal body temperature is your temperature when you're fully at rest. Ovulation may cause a slight increase in basal body temperature.
You'll be most fertile during the two to three days before your temperature rises. By tracking your basal body temperature each day, you may be able to predict when you'll ovulate. This may help you determine when you're most likely to conceive.
If you're hoping to get pregnant, you can use the basal body temperature method to determine the best days to have sex. Similarly, if you're hoping to avoid pregnancy, you can use the basal body temperature method to figure out which days to avoid unprotected sex.
The basal body temperature method alone may not provide enough warning time to effectively prevent pregnancy. Often, people use this method in combination with other fertility awareness-based methods for avoiding pregnancy.
Why it's done
Basal body temperature can be used as a way to predict fertility or as a part of a method of contraception, by helping you gauge the best days to have or avoid unprotected sex.
Tracking your basal body temperature for either fertility or contraception is inexpensive and doesn't have any side effects. Some women may choose to use the basal body temperature method for religious reasons.
The basal body temperature method can also be used to detect pregnancy. Following ovulation, a rise in basal body temperature that lasts for 18 or more days may be an early indicator of pregnancy.
The basal body temperature method is often combined with the cervical mucus method of natural family planning, where you keep track of cervical secretions throughout the course of a menstrual cycle. You might also use an electronic fertility monitor to measure hormone levels in your urine, which can tell you which days you're fertile. This combination of approaches is sometimes referred to as the symptothermal or symptohormonal method.
Using the basal body temperature method to promote fertility doesn't pose any risks.
Likewise, using the basal body temperature method for birth control doesn't pose any direct risks, but it doesn't offer protection from sexually transmitted infections — and it's one of the least effective natural family planning methods. As many as 1 in 4 women — maybe even more — who use fertility awareness-based methods to prevent pregnancy will become pregnant after one year of typical use.
Using the basal body temperature method along with another fertility awareness-based method for birth control may improve the method's effectiveness. But, the method requires motivation and diligence. If you don't want to conceive, you and your partner must avoid having sex or use a barrier method of contraception during your fertile days each month.
How you prepare
Tracking your basal body temperature doesn't require special preparation. However, if you want to use the basal body temperature along with another fertility awareness-based method for birth control, consult your health care provider first if:
- You recently gave birth or stopped taking birth control pills or other hormonal contraceptives
- You're breast-feeding
- You're approaching menopause
Keep in mind that your basal body temperature can be influenced by many factors, including:
- Illness or fever
- Shift work
- Interrupted sleep cycles or oversleeping
- Travel and time zone differences
- Gynecologic disorders
- Certain medications
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.