Robinson & Fleming formula: GA (days) = 8.052 × √CRL(mm) + 23.73. Once a first-trimester scan dates the pregnancy, the due date should not be changed by later scans.
| Gestational age | Typical CRL |
|---|---|
| 7 weeks | 10 mm |
| 8 weeks | 16 mm |
| 9 weeks | 24 mm |
| 10 weeks | 33 mm |
| 11 weeks | 44 mm |
| 12 weeks | 56 mm |
| 13 weeks | 70 mm |
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The crown-rump length (CRL) — the baby's length from head to bottom — measured between about 7 and 14 weeks is the most accurate way to date a pregnancy, to within about 5 days.
Enter the CRL in mm and the scan date to get the gestational age and due date (Robinson & Fleming formula). Add your last period date to see whether the scan date should replace it, using ACOG's rules.
| Crown-Rump Length (CRL) | 45 mm |
| Scan Date | 1 Oct 2026 |
| Gestational age on scan day | 11 weeks 1 day |
| Estimated due date | Wed, 21 Apr 2027 |
| Accuracy | ±5 days (CRL is the most accurate way to date a pregnancy) |
| How far along today | 13 weeks 5 days |
11 weeks 1 day; due Wed, 21 Apr 2027.
Try this example →| Crown-Rump Length (CRL) | 12 mm |
| Scan Date | 1 Oct 2026 |
| Gestational age on scan day | 7 weeks 3 days |
| Estimated due date | Mon, 17 May 2027 |
| Accuracy | ±5 days (CRL is the most accurate way to date a pregnancy) |
| How far along today | 10 weeks 0 days |
7 weeks 3 days.
Try this example →About 55 mm (roughly 45–65 mm between 12+0 and 12+6 weeks).
Before 9 weeks if the scan differs from the LMP date by more than 5 days; between 9 and 13+6 weeks if it differs by more than 7 days.
Ovulation often happens earlier or later than day 14, especially with irregular or long cycles.