What Is Basal Body Temperature?

Basal body temperature refers to your body temperature when you’re completely at rest. For women, monitoring your basal body temperature can provide clues as to when you’re most fertile or have ovulated. That’s because a woman’s resting body temperature changes a tiny bit when she’s ovulating.

If you’re hoping to conceive, having sex just prior to ovulation (release of your egg) increases your chances of conception. Avoiding sex during your “fertile window” decreases your chances of pregnancy — but should not be relied on as your only form of contraception if you are not trying to become pregnant. The “fertile window” is a six day interval of the menstrual cycle ending on the day of ovulation.

Measuring basal body temperature is natural, free, and relatively easy. Learn more about how basal body temperature works — and what to consider if you’re using it as a natural family planning method.

About Basal Body Temperature Tracking

In women of childbearing age, reproductive hormones cause basal body temperature changes around the time of ovulation and after. Your basal body temperature changes are small — usually about half a degree. But if you track your temperature for a few months, you’ll learn to identify your patterns.

Your temperature changes will give you a clue as to when you are ovulating. That’s the time when your body releases an egg, and sperm can fertilize it.

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How to Track Your Basal Body Temperature

Whether you’re trying to conceive or avoid pregnancy, it can be informational to understand how your basal body temperature changes throughout the month:

  • In the days and weeks before you ovulate, your average basal body temperature ranges from 97.0°F to 97.5°F.
  • Immediately before ovulation, your temperature dips about half a degree from your average.
  • After you ovulate, your temperature sharply increases and averages 97.6°F to 98.6°F. It stays there until you get your period, and then the temperature cycle begins again.

Basal body temperature readings are most accurate when you take them first thing in the morning. You should take your temperature as soon as you wake up, before you get out of bed. Also, make sure you don’t eat or drink anything beforehand because doing so affects your temperature.

It’s easiest to see these minor temperature changes with a special basal body thermometer. It’s an oral digital thermometer that measures temperature to the tenth of a degree. Forehead or ear thermometers aren’t accurate enough.

Using Basal Body Temperature to Increase Chances of Conception

You can track your basal body temperature to determine when you’re ovulating. Remember to take your temperature as soon as you wake up, and ideally at the same time each day.

Record your temperature using a chart or a fertility app, starting the first day of your menstrual period. If your menstrual cycle is pretty regular, you’ll begin to see some patterns.

A sudden, slight (half degree) drop in your basal body temperature indicates you’re about to ovulate. This should happen around the same day each month. Take note of which day of the month that temperature change occurs because that’s the time you’re most fertile.

If you want to get pregnant, be sure to have sex every day (or every other day) once your temperature drops. Continue until one day after your temperature rises.

If your cycle is regular and you see a clear pattern, plan to have regular sex based on when you typically ovulate. You can aim to have sex daily (or every other day) up to five days before you expect your temperature to drop. Sperm can live in your reproductive tract for up to five days, so they may still be viable when you ovulate.

While there’s no guarantee of success with this method, it may increase your chances of pregnancy.

Using Basal Body Temperature to Prevent or Delay Pregnancy

Once you learn the approximate day you’re ovulating, you can also use this method to attempt to avoid pregnancy. But since it’s far from foolproof, it’s best to use only if you’re looking to delay (rather than prevent) pregnancy. It is also a good option if you have a religious reason for avoiding birth control.

With natural family planning for birth control, it’s crucial to avoid sex both before and during ovulation. Alternatively, you can use a barrier birth control method — such as a condom or diaphragm during your fertile period. Once ovulation is over, it’s unlikely you’ll conceive.

You can tell when you’re out of your most fertile period because after you ovulate, your basal temperature sharply increases. To be safe, wait at least three to four days after your temperature increase before having unprotected sex. And make sure you stop about a week before you expect your temperature to drop and ovulation to occur again.

Why this method may not be very reliable

Basal body temperature is one of the least effective forms of birth control because so many women have an unpredictable monthly cycle. This means you can’t accurately predict when you will ovulate. And you need to avoid having sex in the days before you ovulate — and before your temperature rises.

Various factors also affect your fertility and temperature. These can include:

How to Increase Effectiveness of Temperature Monitoring

Whether you want to try basal body temperature monitoring to plan or delay a pregnancy, it’s wise to talk to your doctor. They can teach you how to combine the basal body temperature method with other fertility awareness methods. These might include:

  • Checking your cervical mucus. Here you keep track of your cervical secretions throughout your cycle. Your health care provider can describe what it looks and feels like when you’re most and least fertile.
  • Measuring hormone levels in your urine. With this fertility awareness method, you use an electronic fertility monitor. It measures levels of two fertility hormones, estrogen and luteinizing hormone, to tell you when you’re most and least fertile.

When you use all of these fertility awareness methods together, you’ll likely get the best possible picture of your fertility window. And you’ll get a very interesting look at what happens in your body each month.

Sources

American Pregnancy Association. Natural Family Planning: Fertility Awareness Method. LINK

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