Ultrasound Due Date Calculator
Estimate your due date based on precise fetal ultrasound measurements.
Understanding the Ultrasound Due Date Calculator
What is an Ultrasound Due Date Calculation?
An ultrasound due date calculation is the most accurate method for determining a baby’s estimated due date (EDD). Unlike calculations based on the Last Menstrual Period (LMP), which assume a standard 28-day cycle, an ultrasound directly measures the fetus to assess its developmental stage, known as gestational age. This method is particularly reliable and is considered the gold standard by obstetricians.
The accuracy is highest during the first trimester (up to 13 weeks and 6 days), when fetal growth is most consistent among individuals. A measurement called the Crown-Rump Length (CRL) is used during this time. In the second and third trimesters, a combination of measurements (including head, abdomen, and femur length) are used to estimate gestational age. This calculator allows you to use either method to get a precise estimate and to better understand how to calculate due date using ultrasound.
The Formulas to Calculate Due Date Using Ultrasound
This calculator employs established obstetric formulas to convert fetal measurements into gestational age, which is then used to project the due date.
First Trimester: Crown-Rump Length (CRL)
In the first trimester, the gestational age (GA) in days is most accurately calculated using the formula derived from Robinson and Fleming’s research, which is a widely accepted standard. The formula is:
GA (days) = 8.052 * √(CRL_mm * 1.037) + 23.73
Once the gestational age at the time of the scan is known, the Estimated Due Date (EDD) is calculated by the formula:
EDD = Ultrasound Date + (280 days - GA at scan in days)
Second & Third Trimester: Hadlock Formulas
In later stages of pregnancy, a composite gestational age is determined by averaging the age calculated from multiple fetal biometric parameters. This approach, pioneered by Dr. Frank Hadlock, provides a more reliable estimate as different parts of the fetus may grow at slightly different rates. This calculator uses the widely-cited Hadlock formulas for each measurement:
- Biparietal Diameter (BPD):
log10(GA) = 1.3596 + 0.00386(AC) + 0.00064(HC)(Simplified for individual calculation) - Head Circumference (HC)
- Abdominal Circumference (AC)
- Femur Length (FL)
Our calculator computes the GA from each provided measurement and uses the average for the highest accuracy. This is crucial for anyone needing to calculate due date using ultrasound in the second or third trimester. For more details, explore our guide to fetal development.
Variables Table
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| CRL | Crown-Rump Length | mm | 10 – 84 mm (7-14 weeks) |
| BPD | Biparietal Diameter | mm | 20 – 98 mm (12-40 weeks) |
| HC | Head Circumference | mm | 70 – 350 mm (12-40 weeks) |
| AC | Abdominal Circumference | mm | 60 – 385 mm (12-40 weeks) |
| FL | Femur Length | mm | 15 – 78 mm (14-40 weeks) |
Practical Examples
Example 1: First Trimester (CRL)
- Inputs: Ultrasound Date: February 10, 2026; CRL: 50 mm.
- Calculation: Using the CRL formula, a 50mm CRL corresponds to a gestational age of approximately 85 days (12 weeks, 1 day).
- Result: The EDD would be calculated as Feb 10, 2026 + (280 – 85 days) = September 4, 2026.
Example 2: Second Trimester (Multiple Parameters)
- Inputs: Ultrasound Date: May 20, 2026; BPD: 48mm; HC: 175mm; AC: 155mm; FL: 33mm.
- Calculation: The calculator would determine the gestational age from each measurement and average them. This might result in a composite gestational age of approximately 140 days (20 weeks, 0 days).
- Result: The EDD would be calculated as May 20, 2026 + (280 – 140 days) = October 7, 2026. Understanding these steps is key to interpreting your pregnancy health report.
How to Use This Ultrasound Due Date Calculator
- Select the Method: Choose whether you have first-trimester (CRL) or second/third-trimester (BPD, HC, etc.) measurements.
- Enter the Ultrasound Date: Select the exact date your ultrasound scan was performed. This is crucial for accuracy.
- Input the Measurements: Carefully enter the fetal measurements in millimeters as they appear on your ultrasound report. For the multi-parameter method, you need at least two values for a valid calculation.
- Calculate: Click the “Calculate Due Date” button.
- Review Results: The calculator will display your Estimated Due Date, the gestational age at the time of the scan, your estimated conception date, and your current gestational age. You can find more information about what to expect in your first trimester on our blog.
Key Factors That Affect Accuracy
While an ultrasound is the most accurate dating method, several factors can influence the result:
- Timing of the Ultrasound: First-trimester scans (especially 7-11 weeks) are the most precise. Accuracy decreases slightly as the pregnancy progresses due to natural variations in fetal growth.
- Sonographer Skill: The training and experience of the person performing the ultrasound are vital for obtaining accurate measurements.
- Quality of Equipment: High-resolution ultrasound machines provide clearer images and more precise measurements.
- Fetal Position: If the fetus is in a difficult position, it can be challenging to get a perfect measurement of the CRL or other biometric parameters.
- Biological Variation: Every baby grows at a slightly different pace. After the first trimester, genetics and nutrition can cause variations from the average growth curves.
- Multiple Fetuses: Dating a twin or multiple pregnancy can be more complex and may require specialized growth charts. See our guide for multiple pregnancies for more info.
Frequently Asked Questions (FAQ)
1. How accurate is an ultrasound due date?
It is the most accurate method available. A first-trimester ultrasound measuring the crown-rump length (CRL) has an accuracy of about +/- 5 to 7 days. Second-trimester ultrasounds are accurate to within +/- 7 to 10 days.
2. Why is the first-trimester ultrasound the most accurate?
In the early weeks of pregnancy, all fetuses grow at a very predictable rate. After about 14 weeks, individual genetic and environmental factors begin to influence growth, leading to more variability and making dating slightly less precise.
3. What if the ultrasound due date is different from my LMP due date?
This is very common. If the discrepancy is significant (usually more than 7 days in the first trimester), your doctor will typically use the ultrasound date as the official EDD, as it is not subject to recall errors or cycle irregularities.
4. Can my due date change?
Your initial due date, established by your first accurate ultrasound, should not change. Later ultrasounds are used to assess fetal growth and well-being, not to re-date the pregnancy. If a later scan suggests a different date, it indicates the baby is growing larger or smaller than average, not that the date is wrong.
5. What do BPD, HC, AC, and FL mean?
These are standard fetal measurements: BPD (Biparietal Diameter) is the width of the head; HC (Head Circumference) is the measurement around the head; AC (Abdominal Circumference) is the measurement around the belly; and FL (Femur Length) is the length of the thigh bone.
6. What is a composite gestational age?
This is an average gestational age calculated from multiple different measurements (like BPD, HC, AC, and FL). Using a composite average in the second and third trimesters provides a more balanced and accurate estimate than relying on a single parameter.
7. Does this calculator work for IVF pregnancies?
While you can use it to see how measurements correspond to gestational age, IVF pregnancies are most accurately dated from the date of embryo transfer. This calculator is designed for naturally conceived pregnancies dated by ultrasound.
8. What if I only have one or two measurements from my second-trimester scan?
Our calculator can still provide an estimate with two or more measurements. However, the accuracy is highest when all four standard measurements (BPD, HC, AC, FL) are used to create a composite average.
Related Tools and Internal Resources
Explore more of our tools and guides to help you on your pregnancy journey:
- Ovulation Calculator – Pinpoint your most fertile days.
- Pregnancy Weight Gain Calculator – Track your weight gain throughout pregnancy.
- Anatomy Scan: What to Expect – A detailed guide to your 20-week ultrasound.
- Fetal Kick Counter – Monitor your baby’s movements in the third trimester.
- Understanding Your Trimesters – Learn about each stage of pregnancy.
- Your Guide to a Healthy Pregnancy Diet – Nutrition advice for you and your baby.