Why CRL is used for dating
In the first trimester, embryos and fetuses grow at a very similar rate, so their size closely reflects their age. That makes the crown–rump length (CRL) — measured from the top of the head to the bottom of the body on an early ultrasound — the most accurate way to estimate how far along a pregnancy is. National guidelines recommend dating pregnancies by CRL when a scan is done between about 6 and 14 weeks.
The formula
Gestational age (days) = 8.052 × √CRL (mm) + 23.73
Robinson & Fleming, 1975 — widely used in ultrasound software
The calculator applies this formula, rounds to whole days, and adds the remaining days to 280 to give your estimated due date from the scan date.
CRL to weeks: quick reference
| CRL | Gestational age (approx.) |
|---|---|
| 5 mm | 6w 0d |
| 10 mm | 7w 0d |
| 20 mm | 8w 4d |
| 30 mm | 9w 5d |
| 45 mm | 11w 1d |
| 60 mm | 12w 2d |
| 84 mm | 14w 0d |
Different machines and guidelines use slightly different formulas (for example Hadlock’s), so your report may differ by a day.
What to do with the result
If your scan-based date differs from your last-period date, the EDD Calculator Pregnancy applies the rule clinicians use to decide which to keep. If your report gives weeks and days instead of a CRL, use the Ultrasound Due Date Calculator.