Ultrasound Dating Calculator
Estimate gestational age based on fetal ultrasound measurements. Enter your baby's measurements to calculate how far along you are and your estimated due date.
Enter Measurements
Based on CRL measurement
Upcoming Milestones
About Ultrasound Dating
Crown-Rump Length (CRL) is the gold standard for pregnancy dating in the first trimester (7–13 weeks). The measurement from the top of the head to the bottom of the buttocks can predict gestational age with an accuracy of ±3–5 days.
Biparietal Diameter (BPD) measures the diameter across the fetal skull. It's most useful between 14–26 weeks with an accuracy of ±7–10 days.
Head Circumference (HC) becomes increasingly important after 14 weeks. It correlates well with gestational age and is less affected by head shape variations than BPD.
Femur Length (FL) measures the longest bone in the body. It's useful throughout the second and third trimesters but is best combined with other measurements for dating.
Combined measurements (BPD + HC + AC + FL) provide the most accurate dating in the second trimester, with an accuracy of ±5–7 days.
How It Works
Ultrasound dating estimates gestational age by measuring the baby directly, most commonly using crown-rump length (CRL) in the first trimester, or head circumference, abdominal circumference, and femur length later in pregnancy.
First-trimester ultrasound dating (roughly weeks 8 to 13) is considered the most accurate method available, typically accurate within 3 to 5 days, because early fetal growth is remarkably consistent across pregnancies. Dating accuracy decreases as pregnancy progresses, since growth rates increasingly vary between individual babies later on.
When ultrasound dating and LMP-based dating disagree by more than about a week in the first trimester (or more in later trimesters), providers generally use the ultrasound date going forward, since it reflects the baby’s actual measured development rather than an assumption about cycle timing.
Frequently Asked Questions
Why would my due date change based on ultrasound?
Early ultrasound is more accurate than LMP-based dating, especially for women with irregular cycles or uncertain LMP dates. If there is a discrepancy of more than 5–7 days, most providers will adjust the due date to match the ultrasound. This is standard practice and your dating scan is considered the most reliable.
Which ultrasound measurement is most accurate?
CRL measured between 7–13 weeks is the most accurate single measurement, with an error margin of only ±3–5 days. After 14 weeks, a combination of BPD, HC, AC, and FL provides the best accuracy (±5–7 days). In the third trimester, ultrasound dating is less accurate, with margins of ±2–3 weeks.
Can ultrasound dating be wrong?
All ultrasound measurements have inherent error margins. Factors that can affect accuracy include the baby’s position, the operator’s skill, the quality of the ultrasound machine, and biological variation. Early scans (7–13 weeks) are the most reliable.
Why is gestational age counted from LMP instead of conception?
Gestational age is traditionally counted from the first day of the LMP because that date is usually known, whereas the conception date is often uncertain. This means that at “4 weeks pregnant,” you’ve actually been pregnant for about 2 weeks (conception occurs around week 2).
What is a dating scan and when is it done?
A dating scan is an ultrasound performed early in pregnancy (typically 8–14 weeks) to determine gestational age and estimate the due date. It measures CRL, checks for a heartbeat, confirms the number of babies, and assesses the uterus and ovaries. It is considered the most accurate method for pregnancy dating.
- Related Articles
- List Item
Related Tools
⚠️ Medical Disclaimer: This calculator provides estimates based on standard fetal growth curves and formulas. Individual measurements vary. This tool is for educational purposes only and should not replace professional ultrasound dating by a qualified healthcare provider.
