Ultrasound Gestation Period Calculator
Enter an ultrasound crown–rump length of 3–84 mm to estimate gestational age in weeks and days. The result is an approximate pregnancy term, not a clinical dating assessment.
Gestational Age Estimator by Ultrasound Data
Ultrasound Gestation Period Formula
Where:
- \(L\) — embryo crown–rump length (CRL) in millimeters.
- \(A\) — estimated gestational age in weeks before rounding.
- \(T\) — estimated gestational age rounded to a whole number of days.
- \(W\) and \(d\) — completed weeks and additional days shown in the result.
The calculator applies a simplified linear estimate to the entered length, then converts it to weeks and days. This equation is the one used by this calculator; clinical ultrasound dating may use different equations or reference charts.
Ultrasound Gestation Period – Calculation Example
Example: an ultrasound report gives an embryo crown–rump length of 25 mm.
\[ A = 6.5 + 0.15(25) = 10.25\text{ weeks} \]
\[ T = \operatorname{round}(10.25 \times 7) = \operatorname{round}(71.75) = 72\text{ days} \]
\[ 72\text{ days} = 10\text{ weeks} + 2\text{ days} \]
Result: an estimated pregnancy term of 10 weeks 2 days.
How to use the calculator
- Find the crown–rump length (CRL) on your ultrasound report. This is the measurement from the top of the embryo or fetus’s head to its bottom, excluding the legs. The report may abbreviate it as “CRL.”
- Enter that measurement in the Embryo length, mm field. Use millimeters: for example, enter 25 if the report says 25 mm, or 25 if it says 2.5 cm.
- Read the Pregnancy term result as completed weeks plus additional days. An entry of 25 mm produces 10 weeks 2 days.
The accepted input range is 3–84 mm, and decimal measurements are allowed. An entry outside that range produces an error rather than a pregnancy-term estimate. The calculator has no field for a last menstrual period, ultrasound date, or estimated due date.
What an ultrasound gestation estimate means
Gestational age describes how far along a pregnancy is. It is conventionally counted from the first day of the last menstrual period, rather than from fertilization, which typically happens about two weeks later. Thus, a result of 10 weeks 2 days is a gestational-age estimate, not a claim that conception occurred 10 weeks and 2 days ago.
In early pregnancy, a sonographer can measure CRL to help date a pregnancy. A clinician interprets that measurement alongside the scan date, the quality of the image, and any established pregnancy dating. CRL is not the same as gestational-sac diameter, biparietal diameter (BPD), or femur length (FL). Do not enter one of those other measurements here simply because it is also reported in millimeters. For more on the measurement itself, see the Crown Rump Length (CRL) calculator.
Weeks-and-days notation can make estimates easier to compare with a prenatal schedule, but it does not make them exact. If you need to compare a date-based estimate, the Gestational Age Calculator (AOG) and How Many Weeks Pregnant Are You? use different starting information. Their answers may not match this length-based result.
Assumptions and limitations
This calculator uses the fixed equation \(A=6.5+0.15L\) throughout its entire accepted range. Real CRL growth does not follow one straight-line relationship across all early-pregnancy measurements. As a result, the displayed term can differ substantially from an ultrasound report, particularly at larger CRL values. An accepted input is not a guarantee that the resulting age is clinically reliable.
- Check the measurement: use a reported CRL, not an estimated overall fetal length or a value taken from another ultrasound measurement.
- Check the units: convert centimeters to millimeters before entering the number; 1 cm equals 10 mm.
- Allow for uncertainty: image quality, measurement technique, and the dating method all affect estimates. The calculator rounds its computed age to the nearest whole day.
- Use your clinical dating for decisions: if this result conflicts with your ultrasound report or your clinician’s estimated gestational age, ask your prenatal care professional which date to use.
To explore an estimated delivery date once your pregnancy dating has been established, use the Pregnancy Due Date calculator. A due date is itself an estimate and should not be inferred from this calculator’s output without knowing when the ultrasound measurement was taken.
Frequently Asked Questions
How can I calculate the result by hand?
Multiply the CRL in millimeters by 0.15, then add 6.5 to get the calculator’s estimate in weeks. Multiply by 7 and round to the nearest whole day. Finally, divide by 7: the quotient is completed weeks, and the remainder is additional days. For a CRL of 40 mm, the calculation gives 88 days, or 12 weeks 4 days.
What is a normal crown–rump length for my week of pregnancy?
There is no single normal CRL for a given day of pregnancy. CRL changes rapidly in early pregnancy, and measurements vary with fetal position and imaging technique. Compare your result with the reference range used by your ultrasound provider rather than treating this calculator’s estimated age as a growth standard. Ask your prenatal care professional about a measurement flagged as unexpected.
How do I enter a CRL reported in inches?
Multiply inches by 25.4 to convert the measurement to millimeters before entering it. For example, 1 inch equals 25.4 mm. Keep as many decimal places as your report provides; rounding the converted length too early can change the displayed day. If the report includes a millimeter value, use that value directly instead of converting.
Why does the result differ from my last-menstrual-period dating?
The two estimates start with different information: this calculator uses CRL, while menstrual dating uses the date of your last period. Differences can arise from cycle timing, uncertainty about that date, ultrasound measurement variation, and the calculator’s simplified equation. Do not change your established pregnancy dates based on this result alone; ask your prenatal care professional which dating assessment to follow.
Can I use this calculator for twins?
You can enter a reported CRL for either embryo to see a separate mathematical estimate, but do not average their lengths. Dating a multiple pregnancy requires clinical interpretation of the scan and its measurements. If the embryos have different CRLs, ask your ultrasound provider which gestational age has been assigned to the pregnancy.
Can I calculate a due date from the estimated gestational age?
You also need the date the ultrasound was performed. As rough date arithmetic, subtract the gestational age on the scan date from 40 weeks, then add the remaining time to that date. However, this calculator’s age may differ from clinical ultrasound dating, so the resulting due date may be misleading. Use the due date established by your prenatal care professional for planning and care.
Hi is it possible to translate the plugin? maybe using the loco translate plugin
Yes. The plugin has po file for translate
i would like to calculate from embryonic days into WPC