Everything You Need To Know About Basal Body Temperature and Implantation Dip

If you've tried to track ovulation, you might have heard of the basal body temperature (BBT) method, where a person takes daily temperature checks to monitor their menstrual cycle. It's a natural method that requires few tools; however, experts warn this practice isn't always a reliable way to predict ovulation. Plus, beliefs about a slight drop in post-ovulation temperature, referred to as an implantation dip, could cause some to falsely believe they are pregnant.

Read on to learn what BBT is, how it changes during a person's menstrual cycle, and why the implantation dip isn't a trusted way of identifying pregnancy.

What Is Basal Body Temperature and Why Does It Change?

Basal body temperature, a person's lowest natural body temperature, responds to hormonal fluctuations throughout the menstrual cycle, explains Anate Brauer, MD, FACOG, the IVF director at Shady Grove Fertility New York.

"Each cycle starts with resting eggs, a low estrogen level, and a baseline basal body temperature," she says. "As an egg grows and matures, it makes estrogen. Once a peak level of estrogen is reached, ovulation is triggered and the ovaries make progesterone."

Progesterone, she points out, "slightly" increases basal (or base) body temperature. This raised temperature can be used to identify ovulation.

How to Take Basal Body Temperature

"Consistency is key" when it comes to tracking BBT, says Lindsay Kroener, M.D., assistant clinical professor in reproductive endocrinology and infertility at the University of California Los Angeles. Readings should be taken at the same time every day, immediately upon waking, after several hours of consistent sleep. It should be measured before eating, drinking, or any other activity.

In addition, she says a BBT thermometer, which is accurate to the 1/10th degree, should be used. These kinds of thermometers can be found at a typical drugstore.

"When BBT temperature tracking is done, accurate results are dependent on very consistent measurements every day," Dr. Kroener explains, pointing out that changes in temperature during ovulation are typically very small.

A person's temperature will typically rise about 1/2 degree F during ovulation.

Basal Body Temperature Accuracy

While BBT tracking can produce an accurate report of ovulation, experts note this long-practiced method of family planning isn't completely reliable. For one thing, it could be difficult for many to measure their temperature consistently every day. Plus, subtle environmental or lifestyle changes—such as interrupted sleep, changes in sleep patterns, alcohol consumption, emotional stress, infection, jet lag, and having recently stopped taking oral contraceptive pills—could change BBT readings, leading to confusion about the date of a person's ovulation.

In addition, Dr. Brauer points out BBT tracking isn't ideal for those with unpredictable menstrual cycles. "It could be a reliable source for ovulation timing in women with clocklike cycles. However, it may fail women with irregular cycles, such as those with PCOS."

Also, Dr. Brauer says if a person is tracking BBT in an effort to become pregnant through sex, the method itself could be misleading. "The best time to have intercourse when trying to conceive is actually before ovulation since sperm can live in the uterus for up to three to five days, but the egg only lives for 24 hours," says Dr. Brauer. "If you wait until ovulation to have timed intercourse, you have likely missed the boat."

Implantation Dip And Why It's Not Reliable

Implantation dip is a decrease in BBT by at least 0.3 degrees that occurs about a week after ovulation, roughly around the time of implantation, or the point at which a fertilized egg attaches itself to the wall of the uterus.

Some theorize that a drop in BBT is an indicator of implantation; however, there's no peer-reviewed evidence to support this.

"The cause and significance of the implantation dip aren't clear," Dr. Kroener explains. And a post-ovulation temperature decrease can occur in cycles where pregnancy does and does not occur. She calls the implantation dip "a very unreliable indicator of pregnancy, or lack thereof."

Dr. Brauer agrees that an implantation dip should not be used as a way to tell if someone is pregnant. In fact, she points out that an increase in temperature readings would be a more likely sign of early pregnancy. "BBT actually tends to go up during early pregnancy since progesterone increases," she explains.

It's important to note, Dr. Kroener adds, "A BBT chart cannot tell you if you are pregnant and ultimately you will need to take a pregnancy test at the time of a missed period to know for sure."

Basal Body Temperature Alternatives

For those who are tracking BBT in hopes of becoming pregnant, Dr. Kroener notes, while measuring BBT "can be a useful tool," tracking is "often cumbersome."

Ovulation predictor kits, which check for a person's luteinizing hormone (LH) rise in urine, are more reliable and tell users in advance when to expect ovulation, explains Dr. Kroener, making it more convenient to schedule planned intercourse.

And if a person is tracking BBT to avoid pregnancy, there are other, more effective, contraceptive options available. The Centers for Disease Control and Prevention (CDC) identifies a total of 19 hormonal and non-hormonal contraceptive options, including implants, vaginal contraceptive rings, diaphragms, condoms, and sterilization.

The Bottom Line

Basal body temperature, or BBT, may be able to tell a person when they are ovulating but it can be unreliable as various factors can impact it. There is also no concrete evidence that implantation dip, which occurs about a week after ovulation, means implantation has occurred. Experts agree ovulation predictor kits are a better way to track ovulation and a more effective way to time sex for pregnancy. And a BBT chart should never be used to confirm a pregnancy.

Additional reporting by Rachel Gurevich, RN.