🤰 Crown-Rump Length Calculator

What is Crown-Rump Length (CRL)?

CRL stands for crown-rump length — the distance measured from the top of the head (crown) to the buttocks (rump) of an embryo or fetus. It is one of the most fundamental measurements in prenatal care and is the primary method used to determine gestational age during early pregnancy.

The measurement is taken during an ultrasound examination, typically between 6 and 14 weeks of gestation. During this window, fetal growth follows a highly predictable pattern, making CRL one of the most accurate ways to date a pregnancy.

• Embryo (up to 8 weeks after conception): The embryo is developing its major organ systems and the CRL is relatively small, typically under 30 mm.
• Fetus (8–14 weeks): The fetus is growing rapidly and the CRL increases significantly, reaching up to 85 mm or more by the end of the first trimester.

Why is CRL Important? 🤰

Crown-rump length serves several critical clinical purposes:

1. Gestational age dating: CRL provides the most accurate estimate of gestational age in the first trimester, often more reliable than the last menstrual period (LMP) date.
2. Due date estimation: By determining gestational age, doctors can calculate the estimated due date (EDD).
3. Growth monitoring: Serial CRL measurements help monitor whether the embryo/fetus is growing at the expected rate.
4. Chromosomal screening: CRL is used to determine the appropriate timing for nuchal translucency screening (typically between 11–14 weeks).
5. Viability assessment: CRL helps confirm that the pregnancy is viable and progressing normally.

Perfect conditions for estimating your baby's age using CRL include:

• Natural conception
• CRL measurement between 9+0 and 13+6 weeks
• Known last menstrual period
• Regular menstrual cycles
• No use of oral contraceptives or breastfeeding in the previous 2 months

The Robinson & Fleming Equation

Our calculator uses the corrected Robinson and Fleming equation, which is recognized as one of the four most accurate fetal assessment equations by the INTERGROWTH-21st Project, an international study of fetal growth.

This equation was chosen for its simplicity and proven accuracy. It provides reliable results across diverse populations worldwide.

Step-by-Step Calculation Example

Let's calculate the gestational age for a CRL of 55 mm:

1. Multiply CRL by 1.037: 55 × 1.037 = 57.035
2. Take the square root: √57.035 = 7.552
3. Multiply by 8.052: 7.552 × 8.052 = 60.81
4. Add 23.73: 60.81 + 23.73 = 84.54 days
5. Convert to weeks: 84.54 ÷ 7 = 12 weeks and 1 day

CRL Reference Chart

The following chart shows CRL measurements and their corresponding gestational ages, based on the Robinson and Fleming equation:

CRL (mm)	Gestational Age
5	6 weeks 0 days
10	7 weeks 1 day
15	7 weeks 6 days
20	8 weeks 4 days
25	9 weeks 2 days
30	9 weeks 6 days
35	10 weeks 2 days
40	10 weeks 6 days
45	11 weeks 2 days
50	11 weeks 5 days
55	12 weeks 1 day
60	12 weeks 3 days
65	12 weeks 6 days
70	13 weeks 1 day
75	13 weeks 4 days
80	13 weeks 6 days
85	14 weeks 1 day

Discovering the Baby's Age Using CRL and Ultrasound

Once your pregnancy test comes out positive, your next step should be a visit to your OB-GYN. Here's what typically happens:

Early Pregnancy (First Trimester) 🔅

1. The physician asks about the date of the first day of your last menstrual period (LMP)
2. An ultrasound examination is performed to find the CRL and compare it to the crown-rump length chart
3. A full physical examination and detailed medical history are taken

Advanced Pregnancy (Second and Third Trimester) 🔆

1. The physician asks about the last menstrual period
2. Ultrasound measurements shift to head circumference, femur length, and abdominal circumference
3. These values are computed to estimate fetal weight and growth percentiles
4. A full physical examination including fundal height measurement is performed

💡 Important: Visit your gynecologist early on! First-trimester measurements are more accurate for dating purposes. Later measurements are used to assess growth, wellbeing, and rule out diseases or malformations.

How to Use This Calculator

1. Enter the CRL measurement from your ultrasound report
2. Select the appropriate unit (mm, cm, or inches)
3. Click "Calculate Gestational Age"
4. View the estimated gestational age in weeks and days, along with the reference chart

The calculator supports three different units for CRL input, making it compatible with ultrasound reports from different systems and countries.

Frequently Asked Questions