Crown Rump Length Calculator - Gestational Age and EDD

Crown rump length is the distance from the top of an embryo or fetus's head to the bottom of its torso, taken on a first-trimester ultrasound between 6 and 14 weeks. The calculator uses the corrected Robinson and Fleming equation to convert that distance into gestational age in days, weeks plus days, and an optional projected delivery date, so you can confirm a due date from a dating scan.

• • Confirm a due date after a first-trimester scan: Enter the CRL from the ultrasound report and read the gestational age and matching weeks plus days.
• • Reconcile an LMP date with an ultrasound date: Compare the CRL-based gestational age with the last menstrual period to spot a difference of more than 5 to 7 days.
• • Plan antenatal screening milestones: Use the calculated weeks plus days to schedule the nuchal translucency window (11+0 to 13+6) and the anatomy scan (18 to 22 weeks).
• • Estimate a delivery date from an IVF transfer: Pair the CRL reading with the embryo transfer date to confirm growth is on track and anchor an expected due date.

CRL is the most accurate single measurement a sonographer can take in early pregnancy. ACOG, AIUM, and SMFM recommend first-trimester ultrasound for dating because fetal size variation at a given gestational age is smallest between about 8 and 13 weeks.

The reading is usually printed on the scan report in millimetres. The calculator accepts mm, cm, or in, runs the Robinson-Fleming equation, and returns gestational age in plain language like 8 + 4.

For a Naegele-rule due date from a last menstrual period or conception date alongside the CRL-based date, Pregnancy Calculator runs that comparison without a second tool.

The calculator takes the CRL, converts to millimetres, applies the corrected Robinson-Fleming regression, and converts the result into weeks plus days, a trimester label, and a projected delivery date when a scan date is supplied.

• CRL (mm): The measurement in millimetres. Centimetre and inch inputs are converted to mm before the formula runs (1 cm = 10 mm, 1 in = 25.4 mm).
• 8.052: Slope constant from the corrected Robinson-Fleming regression. Multiplies the square root of the scaled CRL.
• 1.037: CRL scale correction that fits the curve to a slightly larger effective measurement.
• 23.73: Intercept constant. Aligns the curve so CRL = 0 corresponds to gestational age just before fertilisation.

When you also enter the date the CRL scan was performed, the calculator projects an expected delivery date by adding 280 days minus the computed gestational-age days to that scan date, which is a useful sanity check against a Naegele-rule due date from the last menstrual period alone.

The Robinson-Fleming equation is the recommended CRL dating equation in the 2014 Napolitano systematic review, which ranked it among the four most accurate fetal-dating equations for CRL between 2 mm and 80 mm.

According to Robinson and Fleming (1975), gestational age in days equals 8.052 times the square root of crown rump length in millimetres multiplied by 1.037, plus 23.73 days.

If the scan record only gives a gestational age in weeks and you want to translate it back into a date, Gestational Age Calculator handles the reverse direction and pairs well with this CRL workflow.

A few definitions and reference values help read a CRL result the same way a sonographer would.

These four concepts frame every CRL-based calculation. The first two are clinical context, the third is the source equation, the fourth is the boundary.

The ACOG / AIUM / SMFM 2017 committee opinion treats first-trimester CRL as the most accurate method to confirm gestational age, with a 5 to 7 day dating accuracy at 8 to 13+6 weeks.

To confirm the CRL-derived delivery date agrees with a Naegele-rule due date, Pregnancy Due Date Calculator runs the LMP-based estimate from the same first day of the last menstrual period so the two numbers can be compared directly.

Read the CRL from the ultrasound report, choose the matching unit, and optionally enter the scan date. The result panel updates as soon as any input changes.

1. 1 Read the CRL from the scan report: Look up the CRL value the sonographer recorded. Most machines print it in millimetres with one decimal.
2. 2 Pick the matching unit: Choose mm, cm, or in so the calculator can convert to millimetres before applying the equation.
3. 3 Add the scan date: If you want a projected delivery date, type the date the ultrasound was performed in YYYY-MM-DD format. Leave blank to skip the EDD.
4. 4 Read the gestational age readout: The primary readout is gestational age in days. The label below shows the same value as completed weeks plus remaining days, the way an obstetric chart writes it.
5. 5 Check the trimester and any data note: Use the trimester label to confirm which phase of pregnancy the CRL implies. If the data note flags a CRL below 2 mm or above 80 mm, switch dating methods or repeat the scan.

Practical use: a dating ultrasound at 10 weeks 4 days shows a CRL of 27 mm. Enter 27 mm, type the scan date, and the calculator returns the matching gestational age and a projected delivery date.

Once the CRL gives a clean gestational age, Conception Calculator can back-calculate the likely conception window so the date lines up with the cycle history recorded in early pregnancy.

Crown rump length is one of the most useful single measurements a clinician can record, and a calculator makes the result consistent and shareable.

• • Fast gestational age readout: Replaces mental arithmetic or chart lookup with a single typed value.
• • Direct match to clinical chart notation: Returns weeks plus days in the same format used in the chart (for example 8 + 4) so the value can be quoted in the antenatal record.
• • Optional delivery date projection: Projects an expected due date from the scan date you enter, which makes it easy to confirm a Naegele-rule due date from the last menstrual period.
• • Built-in unit conversion: Accepts mm, cm, or in so the same calculator works for reports that print CRL in different units.
• • Boundary checks for safer dating: Flags CRLs below 2 mm or above 80 mm, where the Robinson-Fleming equation is less reliable.

The result is a working number for clinical conversations, not a treatment instruction. Share the scan images with the obstetric team first.

ACOG, AIUM, and SMFM continue to recommend first-trimester ultrasound as the most accurate dating method.

After the first trimester, growth questions shift from CRL dating to estimated fetal weight, and Fetal Weight Percentile Calculator compares the ultrasound EFW with INTERGROWTH-21st reference ranges to flag small or large fetuses.

A CRL reading is only as accurate as the measurement behind it. These are the things that move a CRL result up or down.

• • The Robinson-Fleming regression was fitted in a UK cohort in 1975. The 2014 Napolitano review found small differences across ethnic groups, so cross-check with the local growth reference when growth restriction is the question.
• • The calculator is a dating tool, not a screening tool. A normal CRL does not rule out chromosomal conditions, structural anomalies, or later growth issues.

The trimester label is a quick visual cue but is not a substitute for the weeks plus days readout when the question is screening eligibility (for example the 11+0 to 13+6 nuchal translucency window). After 14 weeks the data note reminds the user to switch to biparietal diameter, head circumference, or femur length dating.

According to Napolitano et al. (2014), the corrected Robinson-Fleming equation is among the four most accurate fetal-dating equations for CRL between 2 mm and 80 mm, so this calculator uses it.

According to the American College of Obstetricians and Gynecologists, the American Institute of Ultrasound in Medicine, and the Society for Maternal-Fetal Medicine (2017), ultrasound is the most accurate method to determine or confirm gestational age in the first trimester, so a CRL result should usually take precedence over an LMP-only estimate when the two differ.

When a dating scan disagrees with the LMP-based date and you need to back-calculate a likely last menstrual period, Reverse Due Date Calculator turns a single ultrasound date into the cycle dates that would have produced it.