What this calculator does
In the first trimester, embryos grow at a remarkably consistent rate, which makes crown-rump length the most accurate way there is to date a pregnancy. It is more reliable than the last menstrual period, and it is what dating scans use.
The relationship is not linear. Going from 10 mm to 30 mm covers about 19 days, and the next jump of 30 mm, from 30 to 60, covers about the same 19 days, so each additional millimetre accounts for progressively less time as the pregnancy advances.
The formula
The corrected Robinson and Fleming equation takes the square root of the measurement in millimetres, scales it and adds a constant, giving gestational age in days. A due date follows by counting forward to 280 days.
| Term | Meaning |
|---|---|
| Crown-rump length | The measurement from the top of the head to the bottom of the torso. |
| Gestational age | Age counted from the first day of the last menstrual period, not from conception. |
| Robinson and Fleming | The 1975 equation, in its corrected form, used here and widely in practice. |
The inputs explained
| Field | What to enter |
|---|---|
| Crown-rump length (mm) | Crown-rump length in millimetres from an ultrasound. The equation is most accurate between about 10 and 84 mm. |
| Scan date | The date the scan was performed, used to project a due date. |
When to use it
Dating a pregnancy accurately
First-trimester measurement is the most reliable dating method available.
Checking a scan report
Seeing how the measurement converts makes a report easier to follow.
Understanding a revised due date
Scan dating often shifts a date estimated from the last period.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What gestational age does each measurement give?
Four crown-rump lengths across the useful range.
| Crown-rump length | Estimated gestational age | Gestational age (days) |
|---|---|---|
| 10 mm | 7 weeks 1 days | 49.7 days |
| 30 mm | 9 weeks 6 days | 68.6 days |
| 60 mm | 12 weeks 3 days | 87.2 days |
| 84 mm | 14 weeks 1 days | 98.9 days |
Questions
Why is first-trimester dating so accurate?
Because early growth varies very little between pregnancies. Later on, genetic and environmental differences produce a wide spread in size for the same gestational age, so a third-trimester scan dates a pregnancy far less precisely.
Why is the equation limited to 84 mm?
Beyond roughly 84 mm, around 14 weeks, the fetus curls and straightens enough that a crown-rump measurement stops being reproducible. Other measurements such as head circumference take over after that point.
Does gestational age mean time since conception?
No. It is counted from the first day of the last menstrual period, which is roughly two weeks before conception. This is the universal obstetric convention and is why a 40 week pregnancy involves about 38 weeks of actual development.
How accurate is the due date?
Dating is accurate to within about five days in the first trimester, but the due date itself is an expectation rather than a prediction. Most babies arrive in a window either side of it, and few arrive exactly on it.
For weight gain guidance through the pregnancy, see the pregnancy weight gain calculator. For due date from a last menstrual period, see the due date calculator.