Can You Detect Early Pregnancy With a Basal Body Temperature Chart?

A woman in bed looking at a thermometer, wondering if your BBT chart can tell her if she is pregnant
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You can use your basal body temperature (BBT) to conceive faster, but can a BBT chart predict pregnancy?

Many women read into every little temperature fluctuation. It's part of the two-week wait obsession and the never-ending search for early pregnancy signs. But is the search worthwhile? The answer is yes—and no.

What a Pregnancy BBT Chart Can Show

The entire goal of BBT charting is to detect your most fertile days and aim to have sexual intercourse on those days.

At the very least, your chart should include your daily basal body temperature (taken first thing in the morning), your cervical mucus changes, and the days you’ve done some “horizontal baby dancing.”

Your BBT can’t predict ovulation, but it can show you that you ovulated a few days after it occurs. This means you may not know if you had sex on the “right days” until after ovulation occurs.

However, you can look back on your chart and determine this. You are most likely to conceive if you had sex on the two days preceding ovulation. So, for example, if ovulation occurs on Thursday of a given week, sexual intercourse on Wednesday and Tuesday would give you the best odds to conceive.

Sex on the day of ovulation, the day after ovulation, and three days before ovulation are also likely to lead to pregnancy, though it’s much more likely on the two days before. Of course, having sex when you’re most fertile doesn’t mean you’ll get pregnant, as you already know, if you’ve been doing this a while!

Here’s something you may not know: if you had sex on a day with egg-white quality cervical mucus, you’re much more likely to conceive, even if that doesn’t fall on the day or two before ovulation. It still needs to occur within your fertile window—which is up to five days before ovulation.

Implantation Dips and Triphasic BBT Chart Patterns

You know that your temperature patterns can determine ovulation, but can they indicate embryo implantation and early pregnancy?


Two big things that women look for on a BBT chart are an implantation dip and a triphasic temperature pattern. An implantation dip is a one-day drop in temperature about a week after ovulation. A triphasic temperature pattern is a second temperature increase occurring about one week after ovulation.

Both of these BBT patterns are thought to be signs of increasing progesterone and implantation of the embryo. However, neither is a reliable early pregnancy sign. The majority of the time, an implantation dip is nothing more than a mid-cycle dip in temperature and does not indicate pregnancy. Seeing a triphasic pattern on your BBT chart is slightly more likely to indicate a potential pregnancy, but it is also no guarantee.

Keep in mind that most women who get pregnant get neither an implantation dip nor a triphasic pattern, so if you don’t have one of these pregnancy BBT signs on your chart, don’t worry.

Luteal Phase Length as an Early Pregnancy Sign

The most reliable way to detect pregnancy on a BBT chart takes patience. The old-fashioned method: By waiting to see if your luteal phase—the time between ovulation and your expected period—is longer than usual.

For most women, their luteal phase does not vary by more than a day or two from month to month, even if the length of their menstrual cycle does vary.

For example, a woman’s cycle may vary between being 30 and 35 days, but her luteal phase may consistently be 12 or 13 days long.

If you see that your luteal phase has gone at least one day past the usual length, you might be pregnant. If it goes two days past the longest luteal phase you’ve ever had, the likelihood of being pregnant is even higher. This is a good time to take a pregnancy test.

To be sure, if you reach 18 days past ovulation and you still don’t have your period, chances are very good that you are pregnant. Not many women can wait that long without taking a pregnancy test. Still, it is the strongest early sign of pregnancy detectable with a BBT chart.

A Word From Verywell

Basal body temperature charting is a great way to track your cycles and ovulation patterns. It can also help your doctor detect possible ovulatory infertility. But when it comes to detecting pregnant, BBT charts can only offer small hints. You can't confirm pregnancy with a fertility calendar.

We know how tempting it can be to look for early signs of pregnancy, and how stressful it can be waiting to take a pregnancy test. However, since there are no reliable ways to detect conception without "peeing on a stick," the best way to use your time and energy during the two-week-wait is to focus on self-care and to distract yourself with your life beyond trying to conceive.  


Mayo Clinic. Basal body temperature for natural family planning.

Colombo B, Masarotto G. “Daily fecundability: first results from a new data base.Demogr Res. 2000 Sep 6;3:[39] p.

Triphasic Pattern and Pregnancy: a Statistical