During your first trimester—likely starting at your first prenatal visit—your provider will measure something called the crown to rump length to get a sense of how far along you are and how your pregnancy is progressing. “Accurate pregnancy dating is really important and is best when established as early as possible with the crown rump length,” says Rachel Mandelbaum, MD, an ob-gyn and reproductive endocrinologist with HRC Fertility in California. But why is it so critical—and what exactly is the crown to rump length measurement used for? Read on to learn from experts everything to know about fetal crown to rump length, including how it’s measured and why it matters.

In this article:
Key takeaways
What is crown to rump length in pregnancy?
When is crown to rump length measured during pregnancy?
How is the crown to rump length used in pregnancy?
How is crown to rump length measured?
Crown to rump length by week chart
What can an abnormal crown to rump length measurement mean?
Frequently asked questions
To sum it up

Key Takeaways

Crown to rump length is a measurement used to date a pregnancy and assess baby’s health and development
Crown to rump length is measured via ultrasound and incorporates the length from baby’s head (crown) to their buttocks (rump)
An abnormal crown to rump length may mean the pregnancy was dated incorrectly, or it could indicate a potential prenatal complication if other abnormalities are also found

What Is Crown to Rump Length in Pregnancy?

Crown to rump length (CRL) in pregnancy is exactly what it sounds like: It’s the measurement from the top of baby’s head (i.e. the crown) to the bottom of their buttocks (i.e. the rump), explains Rachael Jones, MSN, APRN, a nurse practitioner and VP of clinical client strategy at WIN, a fertility and family-building benefits provider. The crown to rump length measurement excludes baby’s limbs and yolk sac and instead focuses on the length of the embryo. It’s used to assess baby’s size and development in early pregnancy.

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When Is Crown to Rump Length Measured During Pregnancy?

The crown to rump length measurement is typically taken in early pregnancy, as early as 5 weeks gestation until the end of the first trimester, says Mandelbaum. “At this point, baby becomes too large and begins to uncurl, making it harder to obtain a consistent measurement,” Jones adds. “After that, other biometric markers like head circumference and femur length become more useful.”

How Is the Crown to Rump Length Used in Pregnancy?

The crown to rump length is a main marker of a pregnancy’s health and baby’s wellbeing during the first trimester. It’s also the best way to accurately date the pregnancy and measure baby’s gestational age in the first trimester, Jones says, “especially when the date of conception is unclear or menstrual cycles are irregular.” Having an accurate measurement of this and a due date helps ensure that your prenatal screenings and tests are being scheduled for the right times.

Mandelbaum agrees, adding that there’s a specific and coordinated timeline that’s followed in early pregnancy. “With each week that passes, we expect to see very specific landmarks and growth on ultrasound… We can track this with the crown rump length.”

How Is Crown to Rump Length Measured?

According to Jones, it’s measured via transvaginal ultrasound before 10 weeks of pregnancy, and with an abdominal ultrasound after 10 weeks. For the ultrasound, the technician looks at baby from a side view and uses digital calipers (measurement tools) to measure the crown to the rump in millimeters or centimeters.

Crown to Rump Length By Week Chart

According to Mandelbaum, there are measurement ranges for crown to rump length for each week of pregnancy. Usually, “the crown rump length ranges from less than a centimeter before 6 weeks to potentially over 8 centimeters (80 millimeters) by the end of the first trimester,” she says.

Below, Jones offers an approximation of what baby’s crown to rump length could look like during each week of early pregnancy.

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What Can an Abnormal Crown to Rump Length Measurement Mean?

According to Mandelbaum, an abnormal crown to rump length reading could signal a few things, including inaccurate pregnancy dating or a pregnancy loss. However, usually it needs to be charted over a few weeks to determine root cause of an abnormal measurement.

“If the CRL doesn’t align with the expected size based on the last menstrual period, it may simply indicate a dating discrepancy,” Jones says. If the measurement is smaller than expected, ovulation and implantation might have occurred later than previously thought. “Typically, growth restriction isn’t a concern this early in a viable pregnancy,” Jones adds.

However, a smaller than expected crown to rump length measurement could also signal a chromosomal abnormality—which is usually accompanied by other abnormal tests, such as with the nuchal translucency screening—or an early loss, which would be accompanied by the absence of a fetal heartbeat.

On the other hand, if the measurement is larger than expected, it could again be due to incorrect pregnancy dating, with implantation occurring earlier than previously thought. It could also point to a multiples pregnancy that has not yet been identified. “In very rare cases, a larger-than-expected CRL might be associated with a molar pregnancy or gestational trophoblastic disease,” Jones adds.

Lastly, aside from CRLs being too small or too large, your provider will also ensure it is actually increasing over time. “A lack of growth between ultrasounds spaced 7 to 10 days apart may suggest a nonviable pregnancy and typically warrants follow-up,” Jones says.

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To Sum It Up

Crown to rump length is a way providers track pregnancy early on to ensure baby is growing as expected—but it’s “only one of the many structures needed to establish a healthy early pregnancy,” Mandelbaum says. It can be hard not to stress over the number, particularly if it’s not within a normal range, but Jones explains the crown rump length isn’t a perfect measurement. “Try not to panic…there are many normal reasons a single scan might be off from fetal position to timing of ovulation,” she explains. “A follow-up ultrasound in 7 to 10 days is often all that’s needed to clarify dating or confirm viability.” As you get through the early stages of pregnancy, focus on resting and eating a healthy diet. And for any questions or concerns about your or baby’s health, don’t hesitate to reach out to your provider.

Plus, more from The Bump:

Everything You Need to Know About the First Trimester Screening

Your Guide to Genetic Testing During Pregnancy

When (and How) You Can Find Out Baby's Sex

Fetal Development