CRL Chart: Crown-Rump Length by Week and Day

Crown-rump length, or CRL, is the length of the embryo or fetus from the top of the head to the bottom of the rump, measured in a straight line on a first-trimester scan. Babies grow at almost the same rate at this stage, which makes CRL the most accurate way to date a pregnancy: up to 13+6 weeks, ACOG puts its accuracy at ±5–7 days.

This page has the international INTERGROWTH-21st CRL chart for every day from 8+2 to 15+0 weeks, the reverse table that turns a CRL in millimetres into weeks and days, and a calculator for your due date from a CRL. It also explains why the scan date can differ from your period date, which one is kept, and when CRL stops being used.

What is a normal crown-rump length at each week?

Crown-rump length grows by roughly 1.5 to 1.9 mm a day through the first trimester, so the normal range moves every day rather than every week. On the INTERGROWTH-21st international standard, built from 4,265 healthy pregnancies in eight countries whose dates were known from a certain last period, the median CRL is 21.9 mm at 9+0 weeks, 32.6 mm at 10+0, 43.8 mm at 11+0, 55.6 mm at 12+0, 67.8 mm at 13+0 and 80.6 mm at 14+0. The range widens as the fetus grows: at 12+0, 94 in 100 pregnancies measure between 44.2 and 67.0 mm. A single measurement outside that band is not a diagnosis. In the first trimester the size and the dates measure the same thing, so guidelines treat a CRL that is out of step with the period date first as a dating question, which is how due dates get corrected. The table gives the start of each week; the full chart further down has every day from 8+2 to 15+0.

Source: INTERGROWTH-21st international standard for early fetal size (Papageorghiou et al., Ultrasound Obstet Gynecol 2014). P = centile: 8 in 10 healthy pregnancies measure between P10 and P90, and 94 in 100 between P3 and P97.

CRL calculator: gestational age and due date from CRL

Enter the CRL from your scan report and the date of the scan. The calculator reads the INTERGROWTH-21st dating table, gives the gestational age on the day of the scan, and counts 280 days from the period date that age implies to reach the due date. The second calculator works the other way, from a week and day to the CRL expected at that point.

CRL chart in mm by week and day

Each row is one day, from 8+2 to 15+0 weeks. P50 is the median; 8 in 10 healthy pregnancies sit between P10 and P90, and 94 in 100 between P3 and P97. The chart is the same for every pregnancy: INTERGROWTH-21st publishes one standard, not separate ones for boys and girls or for different countries. For the same weeks in everyday terms, lengths and fruit comparisons included, see baby size by week.

Source: INTERGROWTH-21st international standard for early fetal size (Papageorghiou et al., Ultrasound Obstet Gynecol 2014). P = centile: 8 in 10 healthy pregnancies measure between P10 and P90, and 94 in 100 between P3 and P97. The official downloadable table heads four of its columns as the 5th, 10th, 90th and 95th centiles; recalculated from the paper's own equations they are the 10th, 25th, 75th and 90th, and this page labels them by what they are.

CRL chart by gestational age: how many weeks is a CRL of 20 mm?

Dating runs in the opposite direction, from a measured CRL to an age, and INTERGROWTH-21st publishes a separate table for it. Built from the same pregnancies, it gives the median gestational age for every whole millimetre from 15 to 95 mm and the spread around it.

A CRL of 20 mm is 8+6 weeks. Eight in ten pregnancies measuring 20 mm had a period-based age between 8+3 and 9+3 weeks, and that spread is the real margin of the estimate. If the scan was on 1 October 2026, the due date is 1 October 2026 minus 62 days plus 280 days: 7 May 2027. The same sum for 45 mm (11+1) on the same day gives 21 April 2027. The conception date calculator turns a scan age back into a likely conception date.

Source: INTERGROWTH-21st pregnancy dating standard (Papageorghiou et al., 2014), built from pregnancies dated by a certain last period. Ages are completed weeks+days.

Why the scan date can differ from your period date

Dating from the last period assumes a 28-day cycle with ovulation on day 14. ACOG lists three ways that goes wrong: the first day of the period is misremembered, cycles vary in length, and ovulation moves from month to month. In the reports ACOG cites, only about half of women recalled their last period accurately.

A CRL avoids all three, because it measures the embryo itself. That is why a first-trimester scan so often moves the due date: in one trial cited by ACOG, 40% of women randomised to a first-trimester scan had their due date adjusted because the scan and the period dates were more than 5 days apart.

Which due date is kept: the scan or the period?

The scan, once the difference is big enough to matter, and in the UK as a matter of routine. The rules are written differently in different places:

• UK (NICE and the NHS). NICE guideline NG201 offers every pregnant woman a scan between 11+2 and 14+1 weeks to determine gestational age, and the NHS describes it as the scan that works out your due date.
• US (ACOG). Before 14+0 weeks, the due date is changed to match the scan when the two differ by more than 7 days. Before 9+0 weeks, a difference of more than 5 days is reason enough.
• IVF. The due date comes from the age of the embryo and the transfer date, not from a period or a scan.

Once a due date has been set from the first-trimester scan, it stays. ACOG reserves later changes for rare circumstances, and a first-trimester date is not normally adjusted on the strength of a second-trimester scan. Later scans measure growth against that date instead, which is what fetal growth percentiles describe.

Why your report may not match this chart to the day

Scanners and hospitals do not all use the same CRL chart. NHS screening in England calculates gestational age from the CRL with the charts recommended by the British Medical Ultrasound Society rather than INTERGROWTH-21st. When the INTERGROWTH-21st team compared its dating equation with the two older charts judged least biased, Robinson and Fleming's from Scotland (1973) and Verburg's from the Netherlands (2002–2006), the estimates agreed to within about a day, except above 80 mm, where the gap with the Dutch chart grew past two days beyond 85 mm.

How the measurement is taken matters too. ACOG asks for the mean of three separate measurements where possible, taken in a true midsagittal plane with the spine in view, and the NHS screening programme reports the CRL to one decimal place. A difference of a day or two between your report and this page sits well inside the ±5–7 days that CRL dating is accurate to. Our guide to ultrasound results covers the other abbreviations on the report.

Crown-rump length at 12 weeks

At 12+0 weeks the median CRL is 55.6 mm, with 94 in 100 pregnancies between 44.2 and 67.0 mm. By 12+6 the median is 66.1 mm (53.6 to 78.6 mm). This is the stretch in which most dating scans fall: NICE's window of 11+2 to 14+1 weeks runs, on this chart, from a median CRL of 47.1 mm to one of 82.5 mm.

Scan reports write the age as completed weeks and days, so "12 weeks" on a report means anything from 12+0 to 12+6. The chart's rows use the same notation, which is why they can be matched line for line with your report.

When is CRL no longer used?

From 84 mm. The NHS screening programme dates a pregnancy by head circumference once the CRL is above 84.0 mm, and ACOG notes that beyond 84 mm, about 14+0 weeks, the CRL loses accuracy and second-trimester measurements take over. The INTERGROWTH-21st dating table runs on to 95 mm, which is why the calculator flags any result above 84 mm.

The same window governs one other measurement. Nuchal translucency, part of the combined screening test, is measured only while the CRL is between 45.0 and 84.0 mm. Below 45 mm the NHS programme brings you back for another scan; above 84 mm screening moves to a blood test with dating by head circumference. On the INTERGROWTH-21st chart, 45 mm reads 11+1 weeks and 84 mm reads 14+2. The first trimester screening guide explains that test.

At the small end, the chart stops at 15 mm, about 8+3 weeks. If your scan was earlier than that and the question is whether a heartbeat can be seen yet, the early pregnancy ultrasound guide explains the 7 mm and seven-day rules that NICE sets. A CRL also cannot tell you the baby's sex; our guide to when you can find out the sex covers what can.

When to call your doctor or midwife the same day

A scan measurement is never on this list by itself. A CRL outside the printed range, or a due date that moved, is something to talk through with your antenatal team at the next appointment or by phone, not an emergency.

Frequently Asked Questions

CRL chart — at a glance

Crown-rump length is the first-trimester measurement that dates a pregnancy, accurate to about ±5–7 days up to 13+6 weeks. On the INTERGROWTH-21st standard the median CRL is 21.9 mm at 9+0 weeks, 43.8 mm at 11+0, 55.6 mm at 12+0 and 80.6 mm at 14+0, and the normal range between the 3rd and 97th centiles widens as the fetus grows. Read the other way, a CRL of 20 mm is 8+6 weeks and 45 mm is 11+1, and the due date is the scan date minus that age plus 280 days. When the scan and the period disagree, the scan usually sets the date: in the UK by routine, in the US once the gap is more than 7 days. From 84 mm dating moves to head circumference, and nuchal translucency is measured only while the CRL is between 45 and 84 mm.

Read next

Sources

• Papageorghiou et al., Ultrasound in Obstetrics & Gynecology 2014 — International standards for early fetal size and pregnancy dating based on crown–rump length (INTERGROWTH-21st)
• INTERGROWTH-21st — Fetal size in early pregnancy: CRL centile tables, 8+2 to 15+0 weeks
• INTERGROWTH-21st — Pregnancy dating: gestational age by CRL, 15 to 95 mm
• NICE NG201 — Antenatal care: recommendations (1.2.14, the 11+2 to 14+1 week scan)
• NHS Fetal Anomaly Screening Programme handbook — CRL 45.0 to 84.0 mm, and dating by head circumference above 84.0 mm
• ACOG Committee Opinion 700 — Methods for Estimating the Due Date
• NHS — 12-week scan (the dating scan)
• NHS — Ectopic pregnancy

Baby Novum is an informational and tracking tool, not a medical device, and does not provide medical diagnosis or treatment. Always consult your doctor or midwife about your pregnancy.