Accurately calculate fetal gestational age using your Last Menstrual Period (LMP) or ultrasound measurements (CRL, BPD, HC, FL) to estimate your due date and track fetal development.
This calculator is based on Naegele's rule, assuming ovulation occurs around day 14 of the menstrual cycle.
Please note that the calculation results are for reference only. The accurate due date should be confirmed by a doctor through ultrasound examination.
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Still confused by the data on your ultrasound report? Based on clinically validated formulas, this tool converts fetal ultrasound biometric measurements (such as Crown-Rump Length (CRL), Biparietal Diameter (BPD), Head Circumference (HC), and Femur Length (FL)) or your Last Menstrual Period (LMP) date into an accurate gestational age in weeks. The core principle of gestational age calculation is establishing a mathematical mapping model through the statistical relationship between measurements and gestational age growth curves.
What data is needed to calculate gestational age?
You need at least your Last Menstrual Period (LMP) date or any single ultrasound measurement (such as CRL). Combining multiple measurements can improve accuracy.
What is the priority of indicators for different gestational weeks in the ultrasound method?
In the first trimester (≤14 weeks), CRL is the most accurate. In the second and third trimesters, a combination of BPD, HC, and FL is recommended. Femur length (FL) has a sensitivity of ±3 days for gestational age, and combining multiple indicators reduces the impact of single measurement errors.
In the first trimester, the CRL measurement error should be kept within ±5mm. When combining multiple indicators in the second and third trimesters, if any data point is abnormal (e.g., FL is significantly short), it is recommended to re-measure or consult a doctor. The LMP method requires accurately recalling the start date of your last period; for those with irregular cycles, the error margin can be up to 1-2 weeks.
Clinical recommendation: At 22 weeks of gestation, a typical Femur Length (FL) is 40±3mm, corresponding to a gestational age error of about ±4 days. If an FL of 37mm is calculated alone, it might yield 22.5 weeks, but cross-referencing with an HC of 195mm adjusts it to 21.8 weeks—demonstrating the value of multi-parameter validation. Results should be interpreted comprehensively within a clinical context, as isolated data can be misleading.
