Crown-Rump Length Calculator
Estimate gestational age from CRL measurement.
For Informational Purposes Only
Enter a CRL value to see the results.
In the early stages of pregnancy, one of the most exciting and important milestones is the first ultrasound. During this scan, a key measurement taken is the Crown-Rump Length (CRL), which is the most accurate way to determine a baby's gestational age and, consequently, the estimated due date (EDD). This CRL calculator is designed to help expectant parents and students understand how this measurement translates into a pregnancy timeline.
This tool is for educational purposes only. The official gestational age and due date must be determined by a qualified healthcare professional who performs the ultrasound.
What is Crown-Rump Length (CRL)?
Crown-Rump Length is the measurement of the length of a fetus from the top of its head (the crown) to the bottom of its buttocks (the rump). It is typically measured in millimeters (mm).
This measurement is used during the first trimester of pregnancy, generally between 7 and 14 weeks. During this period, fetuses of the same age tend to grow at a very consistent and predictable rate. The CRL is a straight-line measurement and does not include the yolk sac or the limbs (legs), as they are still developing and curled up against the body. This consistency makes the CRL the single most accurate predictor of gestational age in early pregnancy.
Why is CRL Measurement So Important?
- Accurate Dating: It provides the most precise estimation of the gestational age, which is crucial for scheduling future tests and monitoring the baby's growth. It can be more accurate than dating based on the Last Menstrual Period (LMP), especially if a woman has irregular cycles.
- Confirming Viability: A CRL measurement combined with the presence of a fetal heartbeat confirms that the pregnancy is viable and progressing as expected.
- Detecting Multiple Pregnancies: An early ultrasound can identify if there is more than one fetus.
How is Gestational Age Calculated from CRL?
Obstetricians and sonographers use established formulas and charts to convert the CRL measurement into a gestational age. There are several different formulas, but one of the most widely cited and used is the Robinson & Fleming formula. This calculator uses that formula:
Gestational Age (in days) = 8.052 × √CRL (in mm) + 23.73
This formula takes the square root of the CRL measurement in millimeters, multiplies it by a constant, and adds another constant to arrive at the total gestational age in days. Our calculator then converts this number into the more familiar format of weeks and days.
How to Use the Calculator
- Find the CRL Value: Look at your first-trimester ultrasound report. The CRL should be listed, usually in millimeters (mm).
- Enter the Value: Input the CRL measurement into the calculator. The valid range is typically between 5mm (around 6 weeks) and 84mm (around 14 weeks), after which other measurements like Biparietal Diameter (BPD) become more accurate for dating.
The calculator will instantly show you:
- Estimated Gestational Age: Your pregnancy's age in weeks and days.
- Estimated Due Date (EDD): A projected due date based on the calculated gestational age.
After the First Trimester
As the pregnancy progresses into the second trimester, the baby begins to uncurl, and individual variations in growth become more pronounced. At this point, the CRL is no longer the most accurate measurement. Instead, sonographers will measure other parameters to assess growth and well-being, including:
- Biparietal Diameter (BPD): The diameter of the baby's head.
- Head Circumference (HC): The circumference of the baby's head.
- Abdominal Circumference (AC): The circumference of the baby's abdomen.
- Femur Length (FL): The length of the thigh bone.
These measurements are used to estimate fetal weight and ensure the baby is growing along a healthy curve, but the initial due date established by the first-trimester CRL is not usually changed.
Understanding your ultrasound report and the meaning of measurements like the Crown-Rump Length can make you a more informed and engaged participant in your prenatal care. Use this calculator as a guide to better understand the very first snapshots of your growing baby.
Enter values to see the results.
In the early stages of pregnancy, one of the most exciting and important milestones is the first ultrasound. During this scan, a key measurement taken is the Crown-Rump Length (CRL), which is the most accurate way to determine a baby's gestational age and, consequently, the estimated due date (EDD). This CRL calculator is designed to help expectant parents and students understand how this measurement translates into a pregnancy timeline.
This tool is for educational purposes only. The official gestational age and due date must be determined by a qualified healthcare professional who performs the ultrasound.
What is Crown-Rump Length (CRL)?
Crown-Rump Length is the measurement of the length of a fetus from the top of its head (the crown) to the bottom of its buttocks (the rump). It is typically measured in millimeters (mm).
This measurement is used during the first trimester of pregnancy, generally between 7 and 14 weeks. During this period, fetuses of the same age tend to grow at a very consistent and predictable rate. The CRL is a straight-line measurement and does not include the yolk sac or the limbs (legs), as they are still developing and curled up against the body. This consistency makes the CRL the single most accurate predictor of gestational age in early pregnancy.
Why is CRL Measurement So Important?
- Accurate Dating: It provides the most precise estimation of the gestational age, which is crucial for scheduling future tests and monitoring the baby's growth. It can be more accurate than dating based on the Last Menstrual Period (LMP), especially if a woman has irregular cycles.
- Confirming Viability: A CRL measurement combined with the presence of a fetal heartbeat confirms that the pregnancy is viable and progressing as expected.
- Detecting Multiple Pregnancies: An early ultrasound can identify if there is more than one fetus.
How is Gestational Age Calculated from CRL?
Obstetricians and sonographers use established formulas and charts to convert the CRL measurement into a gestational age. There are several different formulas, but one of the most widely cited and used is the Robinson & Fleming formula. This calculator uses that formula:
Gestational Age (in days) = 8.052 × √CRL (in mm) + 23.73
This formula takes the square root of the CRL measurement in millimeters, multiplies it by a constant, and adds another constant to arrive at the total gestational age in days. Our calculator then converts this number into the more familiar format of weeks and days.
How to Use the Calculator
- Find the CRL Value: Look at your first-trimester ultrasound report. The CRL should be listed, usually in millimeters (mm).
- Enter the Value: Input the CRL measurement into the calculator. The valid range is typically between 5mm (around 6 weeks) and 84mm (around 14 weeks), after which other measurements like Biparietal Diameter (BPD) become more accurate for dating.
The calculator will instantly show you:
- Estimated Gestational Age: Your pregnancy's age in weeks and days.
- Estimated Due Date (EDD): A projected due date based on the calculated gestational age.
After the First Trimester
As the pregnancy progresses into the second trimester, the baby begins to uncurl, and individual variations in growth become more pronounced. At this point, the CRL is no longer the most accurate measurement. Instead, sonographers will measure other parameters to assess growth and well-being, including:
- Biparietal Diameter (BPD): The diameter of the baby's head.
- Head Circumference (HC): The circumference of the baby's head.
- Abdominal Circumference (AC): The circumference of the baby's abdomen.
- Femur Length (FL): The length of the thigh bone.
These measurements are used to estimate fetal weight and ensure the baby is growing along a healthy curve, but the initial due date established by the first-trimester CRL is not usually changed.
Understanding your ultrasound report and the meaning of measurements like the Crown-Rump Length can make you a more informed and engaged participant in your prenatal care. Use this calculator as a guide to better understand the very first snapshots of your growing baby.