Ultrasound Due Date Calculator
Based on ultrasound dating
Ultrasound Due Date Calculator
Enter your scan date and the gestational age your sonographer measured on that day to get an ultrasound-based due date, current gestational age, and progress through pregnancy. Add your last period date too, and it’ll check whether ultrasound or LMP dating should be used, using the same rule your care team does.
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Ultrasound vs. LMP dating
- LMP-based due date—
- Difference between the two methods—
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How this is calculated, and the ACOG dating rule
Ultrasound-based due date. The due date is calculated as the scan date plus the number of days remaining to reach 280 days (40 weeks) of gestational age — in other words, working forward from the gestational age your sonographer assigned on the day of the scan.
Today’s gestational age is the age at your scan plus the number of days that have passed since then, assuming ultrasound dating is being used going forward.
Ultrasound vs. LMP dating. When both dates are entered, this uses the rule from ACOG Committee Opinion 700 (Methods for Estimating the Due Date) to check whether the two methods agree closely enough, or whether ultrasound dating should take precedence. The allowed discrepancy before switching to ultrasound dating narrows the earlier the scan is done:
- Before 9 weeks 0 days: more than 5 days’ difference
- 9 weeks 0 days – 13 weeks 6 days: more than 7 days
- 14 weeks 0 days – 15 weeks 6 days: more than 7 days
- 16 weeks 0 days – 21 weeks 6 days: more than 10 days
- 22 weeks 0 days – 27 weeks 6 days: more than 14 days
- 28 weeks 0 days or later: more than 21 days
Earlier scans are more accurate for dating than later ones, which is why the tolerance is tightest in the first trimester and loosens as pregnancy progresses — fetal size varies more between individuals later on, making late-pregnancy scans less reliable for dating (though still useful for growth assessment).