Enter a femur length (FL) measurement from an ultrasound report to view an estimated gestational age range. The result is informational and may vary depending on scan quality, fetal position, and clinical context.
Calculate from Femur Length (FL)
This calculator uses a quadratic (non-linear) femur-length regression approach commonly used in obstetric ultrasound biometry references. It accepts a broad FL entry range because reports and tools may display different values, but it only displays a gestational-age result when the computed estimate lands within a practical window of about 14–40 weeks.
Results
Understanding Femur Length and Ultrasound Gestational Age Estimation
Many people search for a femur length to gestational age estimate after receiving an ultrasound report that includes fetal biometry. Femur length (often abbreviated as FL) measures the length of the fetal thigh bone and can contribute to an overall picture of fetal size. In clinical practice, ultrasound dating and growth assessment commonly consider multiple measurements together, because each measurement can be influenced by fetal position, image quality, and normal biological variation.
This page is designed to support an on-page fetal femur length calculator by explaining what FL can and cannot indicate. When an ultrasound report lists FL, it is typically one of several parameters alongside head and abdominal measurements. Those measurements can be integrated to estimate gestational age and to evaluate whether growth appears consistent with the expected timeline. If you are comparing results across websites, it is normal to see differences, because different tools may use different models, rounding rules, or reference standards.
What this calculator does
The calculator takes FL in millimeters as input, converts it to centimeters, and applies a quadratic regression equation to estimate gestational age. Quadratic models are often used in pregnancy ultrasound measurements because fetal growth is not perfectly linear throughout pregnancy. The result is displayed as weeks and days for readability.
Practical output rule: this tool accepts broad entries (10–90 mm) to match how values appear on reports and other sites, but it displays a weeks-and-days estimate only when the computed result falls within a practical dating window of about 14–40 weeks. Outside that window, the relationship between FL and gestational age can be less suitable for “dating” in a general-audience calculator.
Why results can vary between tools
Ultrasound-based gestational age estimation is sensitive to method choice. Some calculators are designed for “dating” (estimating gestational age), while others focus more on fetal growth assessment (comparing size to reference charts). A growth-focused tool may show percentiles or interpret measurements relative to a gestational age already established by earlier dating. A dating-focused tool may output a single weeks-and-days estimate based on one measurement, which can look precise even though the underlying uncertainty can increase later in pregnancy.
It can also help to remember that ultrasound measurements are not taken in isolation. Clinicians typically interpret the complete ultrasound report and the clinical timeline together. If an estimate differs from your established due date, that does not automatically mean anything is “wrong.” It may reflect a difference in reference model, variation in fetal proportions, or the fact that ultrasound precision can change with gestational age.
How to think about the due date shown here
This page includes an estimated due date based on the gestational-age estimate and today’s date. It is intended as an educational reference, similar to how a general pregnancy due date calculator converts weeks into a calendar date. In clinical settings, the “best” due date is commonly based on early dating information and the overall clinical picture.
Helpful related tools on this site
If you are exploring pregnancy timelines and ultrasound results, these pages may also be useful depending on your question: gestational age calculator, CRL calculator (often used earlier in pregnancy), fundal height calculator, mean sac diameter calculator, and conception date calculator. Each tool has different assumptions and is suited to different stages and use cases.
Government references for pregnancy and prenatal care information
For general, non-personalized pregnancy information from government and public health sources, you can review: CDC pregnancy resources, Office on Women’s Health (HHS), and MedlinePlus (NIH). These references can provide background on prenatal care, routine ultrasounds, and common questions during pregnancy.
Frequently Asked Questions
Tap a question to expand the answer.
Why does the calculator sometimes not display a gestational age result?
This page only displays a weeks-and-days estimate when the computed result lands within a practical window of about 14–40 weeks. Outside that range, using femur length alone for dating can be less suitable in a general-purpose calculator.
My femur length is 90 mm. Why do some sites show a week estimate anyway?
Many websites will still output a number because the equation continues to produce a value mathematically. That does not necessarily mean the estimate is appropriate for general “dating” near or beyond term.
Is femur length used for dating or growth assessment?
Femur length can support both contexts. In many clinical settings, gestational age is interpreted using multiple ultrasound measurements and the clinical timeline, while femur length may also contribute to growth assessment when gestational age is already established.
Why can accuracy be lower in the third trimester?
As pregnancy progresses, normal biological variation in fetal size tends to increase. That can widen the range of “typical” measurements at the same gestational age, especially when using a single measurement.
Why might my clinic’s estimate differ from this tool?
Clinics may use different reference charts or combined biometry formulas rather than femur length alone. Earlier scans, menstrual history, and the full ultrasound report may also be considered.
Does this calculator replace my clinical due date?
No. This page provides an educational estimate. Clinical due dates are typically determined using the overall clinical picture and often prioritize early dating information when available.
What unit should I enter for femur length?
Enter femur length in millimeters (mm). If your report is in centimeters (cm), multiply by 10 to convert to mm before entering. Unit mix-ups are a common reason for unexpected results.
When might it help to discuss results with a healthcare professional?
If you notice a large difference compared with your established dating, if your report recommends follow-up, or if you have questions about growth and measurement interpretation, a licensed healthcare professional can provide context using the full ultrasound report and clinical timeline.
This page is for informational and educational purposes only. It does not provide medical advice, diagnosis, or treatment. Ultrasound measurements and gestational-age estimates can vary based on scan quality, technique, timing, and individual factors. For personal medical questions or interpretation of an ultrasound report, consult a licensed healthcare professional.