How Basal Body Temperature Charting Can Help You Get Pregnant

How to Take Your Basal Body Temperature & Detect Your Most Fertile Time

Woman taking her body basal temperature
When taking your body basal temperature, it's important that you don't sit up or move around before you take it. Otherwise, it will throw off your chart. sozaijiten / Datacraft / Getty Images

Your basal body temperature is your body's temperature at complete rest. When you ovulate, your body basal temperature rises slightly. You can use this information to detect ovulation and get pregnant faster

Charting your basal body temperature is a relatively easy and inexpensive way to track ovulation. All you need is an accurate thermometer, a fertility charting app or program (there are lots of free ones), and some patience.

What Is Your Basal Body Temperature? What Does It Have to Do With Getting Pregnant? 

While you might think of your temperature as either being normal or feverish, there are a lot of normal variations in between. Our body temperature changes based on movement, how much sleep we've had, the time of day, exercise, and hormonal changes. 

After you ovulate, the hormone progesterone begins to rise. Progesterone causes a slight increase in temperature. You can detect this change by charting your body basal temperature.

The shift upwards caused by ovulation is at least four-tenths of a degree. For example, 98.4 is four-tenths higher than 98.0. If this upward change stays around for at least three days, you can be pretty sure that ovulation has occurred on the day before the temperature rise.

To get pregnant, you need to have sex before the shift occurs. Your chart can help you see when you ovulate each month, so you can time sex for pregnancy better.

(For the best results, you should also look for other ovulation signs and mark them on your fertility calendar. More on this below.)

If this sounds complicated, don't worry. Most fertility charting programs do the hard work -- like figuring out when you ovulated and when you might ovulate next month -- for you. All you have to do is take your temperature correctly and input your information.

How to Take Your Body Basal Temperature Correctly

Because the shift you're looking for is so small, it’s important to take your temperature at the same time every morning before you get up or move.

You will need a basal thermometer that displays at least one-tenth of a degree. Just about every thermometer you can buy at the pharmacy does this. 

Once you've got the right kind of thermometer, here's what to do next:

  1. Before you go to bed at night, place a thermometer within your reach by your bed. You need to be able to reach it without sitting up or moving around. Getting out of bed with throw off your temps and skew your results. 
  2. If you're using a mercury thermometer, shake it down before bed. Don't plan to shake it down in the morning, as that can increase your body temperature.
  3. Set your alarm the same time every day. Yes, even on the weekends. An extra hour or two of sleep will throw your chart off. Aim to take your temp within the same 30-minute window every morning.
  4. When you wake up, reach for the thermometer and take your temp. Do not go to the bathroom first! Don't sit up! 
  5. Take your temp orally or vaginally. There isn't a difference in which method you use, as long as your consistent. Orally is the easiest (and more comfortable for most people. But if you sleep with your mouth open, you may want to try the vaginal route.  
  6. Follow the directions for your thermometer to get the best reading. If you’re using a mercury thermometer, make sure you leave it in place long enough to get a final reading. That may take up to four or five minutes.
  1. After you take your temperature, write it down. Keep a notepad and pen by your bedside to make this simple. Or, you may input your result right into your smartphone, if you're using a fertility app. Some BBT thermometers come with a memory function, which is nice.

Here are a couple more tips:

  • If you need to wake up extra early, or later than usual, take your temperature as you always do. Mark the difference in time on your chart. Yes, it won't be as accurate, but one day off isn't a big deal as long as your consistent most of the time.
  • You need to have slept at least four straight hours for your temperature to be accurate. If you get up often at night or have trouble sleeping, this can throw off your results. You should take your temperature anyway, and note that your sleep was interrupted on your chart.

Getting Started With Charting Your Basal Temperature

Taking your temperature is only part one. You can't do much with just one body basal temperature. You need a series of them, and you need to record them somewhere. 

The very best way is with a fertility charting app or computer program. You won't have to worry about whether your temperature is sustained long enough or not, or spend time calculating when you might be most fertile the next day. 

The software will do that for you! 

If you're an old fashioned kind of girl, you could totally chart your own temps on graph paper.

I highly recommend reading Toni Weschler's book Taking Charge of Your Fertility (William Morrow Paperbacks; Rev Upd edition (July 7, 2015)). You'll find lots of examples and instructions on interpreting your chart. 

Why You Should Also Track Cervical Mucus Changes on Your Chart

The great thing about body basal temperature charting is you can share your charts with your doctor. If it seems you're not ovulating, or if your luteal phase looks too short, you can share this information easily with your doctor.

The problem with only looking at your body basal temps is that it only tells you if you ovulated after it happened. But you need to have sex before ovulation to get pregnant. 

You should also consider checking for changes in your cervical mucus. This is also really easy to do, and most fertility charting apps allow you to put this information into your chart.

When you see fertile cervical mucus -- it resembles raw egg whites -- this is the best time to have sex to conceive

For bonus points, you can even track changes in your cervical position. (Yes, you can check your cervix!) 

Possible Pitfalls to Body Basal Temperature Charting

Body basal temperature charting is not for everyone. 

It may not be for you if...

  • You work the night shift.
  • You have to get up often in the night with young children. 
  • Your can't get up at around the same time each day.
  • It stresses you out. 

Charting may be all the rage on fertility forums, but you can totally get pregnant without it. You can look for other ovulation signs, or just have sex frequently. 

Also, some women won't see a sustained rise in temperature, even if they do ovulate. However, because this affects a small percentage of the population, you should mention it to your doctor.