Due Date by Ultrasound Calculator
Get your due date from a dating scan, and see whether it should replace the date from your last period.
Look for “GA” or “gestational age” on your scan report, for example 11w 1d or 11+1.
Head to bottom, from an early scan. Used for dating up to 84 mm (about 13 weeks 6 days).
From a scan after about 14 weeks. Fill in the ones on your report; all four gives the best answer.
Add it to check whether the scan should change your due date. Clear it if you don't know it.
Only have your period date, a conception date or an IVF transfer? Use the pregnancy due date calculator or the conception & IVF due date calculator. Your midwife or doctor sets your official due date.
How your due date by ultrasound is worked out
The scan's weeks and days, plus the days left to 40 weeks, then a check against your period date.
- 1
Measure the baby
Report says
= 11w 1d - 2
Add the days left
280 − 78 = 202 d
scan + 202 d
= 2 May 2027 - 3
Compare with period
period: 12w 4d
10 days apart - 4
Apply ACOG rule
limit 7 days
→ use the scan date
Read more: the formulas, a worked example and the ACOG table
A dating scan measures the baby and gives how far along you are on the day of the scan. Count on to 40 weeks (280 days) from there, and you have the due date. If you also know your last period, the two dates are compared, and the scan date replaces the period date only when the gap is bigger than the limit for that stage of pregnancy.
The formula
due date = scan date + (280 − days pregnant at the scan)
days pregnant from crown–rump length (Robinson–Fleming) = 8.052 × √(CRL in mm) + 23.73
gap = days pregnant by the scan − days pregnant by your period, on the scan day. Use the scan date if the gap is more than the limit below.
For head and bone sizes (after about 14 weeks) we use Hadlock's formulas, with each size in cm and the answer in weeks: head width (BPD) 9.54 + 1.482 × BPD + 0.1676 × BPD²; head circumference (HC) 8.96 + 0.540 × HC + 0.0003 × HC³; tummy (AC) 8.14 + 0.753 × AC + 0.0036 × AC²; thigh bone (FL) 10.35 + 2.460 × FL + 0.170 × FL². With all four: 10.85 + 0.060 × HC × FL + 0.6700 × BPD + 0.1680 × AC. With fewer, we average the ones you entered.
Worked example
Your last period started on 1 January 2026, and a scan on 28 March 2026 measured a crown–rump length of 45 mm. 8.052 × √45 + 23.73 = 8.052 × 6.708 + 23.73 = 77.74, so 78 days, or 11 weeks 1 day. The due date by the scan is 28 March + (280 − 78) = 28 March + 202 days = 16 October 2026.
By your period, 28 March is 86 days after 1 January, so 12 weeks 2 days, and the period due date is 1 January + 280 days = 8 October 2026. The gap is 78 − 86 = −8 days: the baby measures 8 days smaller. At 12 weeks 2 days the limit is 7 days, and 8 is more than 7, so use the scan date, 16 October 2026. If the scan had said 12 weeks 0 days, the gap would be 2 days and you'd keep 8 October.
The calculator above starts with a similar example from today's date: a scan a week ago showing 11 weeks 1 day, and a period 95 days ago (12 weeks 4 days on the scan day). That's a 10-day gap, over the 7-day limit, so the scan date is used.
When the scan date replaces the period date
The American College of Obstetricians and Gynecologists (ACOG Committee Opinion 700, Methods for estimating the due date, May 2017) suggests changing the due date to the scan's when the difference is more than:
| Pregnancy length at the scan | Measured by | Change the date if the gap is over |
|---|---|---|
| up to 8w 6d | CRL | 5 days |
| 9w 0d – 13w 6d | CRL | 7 days |
| 14w 0d – 15w 6d | BPD, HC, AC, FL | 7 days |
| 16w 0d – 21w 6d | BPD, HC, AC, FL | 10 days |
| 22w 0d – 27w 6d | BPD, HC, AC, FL | 14 days |
| 28w 0d and later | BPD, HC, AC, FL | 21 days |
We pick the row by how far along you were by your period on the scan day. Some other points from the same guidance:
- The earliest good scan sets the date. Once a pregnancy is dated, later scans are used to check growth, not to change the due date.
- Crown–rump length is used up to 84 mm (13 weeks 6 days). After that, head and bone sizes are used.
- A pregnancy with no scan before 22 weeks 0 days is called “suboptimally dated”.
- IVF pregnancies are dated from the embryo's age at transfer, not from a scan. Use the conception & IVF due date calculator.
Sources: ACOG Committee Opinion 700 (2017); Robinson HP, Fleming JE, Br J Obstet Gynaecol 1975; Hadlock FP et al., Radiology 1984. Scan machines may use other published charts (for example Hadlock or INTERGROWTH-21st for CRL), so your report can differ from this by a day or two. Written 11 October 2026.
For information only, not medical advice. Your midwife or doctor will confirm your due date and care.
Common questions
How is a due date worked out from an ultrasound?
Take how far along the scan says you were on the day of the scan, in days, and subtract it from 280 (40 weeks). Add what is left to the scan date. A scan on 28 March showing 11 weeks 1 day (78 days) gives 28 March + 202 days = 16 October.
Is the ultrasound due date more accurate than my period date?
An early scan is usually more accurate, because it measures the baby instead of assuming when you ovulated. The crown–rump length taken before 14 weeks is the most exact way to date a pregnancy. Later scans are less exact, because babies grow at different rates.
When should the due date be changed to the scan date?
ACOG suggests changing it when the scan and period dates differ by more than 5 days up to 8 weeks 6 days, 7 days from 9 weeks to 15 weeks 6 days, 10 days from 16 to 21 weeks 6 days, 14 days from 22 to 27 weeks 6 days, and 21 days from 28 weeks.
What is crown–rump length (CRL)?
It is the length of the baby from the top of the head (crown) to the bottom (rump), measured on an early scan. It is used for dating up to 84 mm, about 13 weeks 6 days. With the Robinson–Fleming formula, a CRL of 45 mm means about 11 weeks 1 day.
Will my due date change at my 20-week scan?
Usually not. Once a pregnancy has been dated by an early scan, the date is normally kept, and later scans are used to check growth instead. If there was no earlier scan, the date may be changed when the gap is more than 10 days between 16 and 21 weeks 6 days.
Related calculators
Made by Zulu Calculator, part of AlphaZulu LLC. Formulas are shown in full and rule figures are checked against official sources; see how we build calculators.