Ultrasound Gestational Age Calculator
Enter your scan date and fetal biometry (BPD, HC, AC, FL) to estimate gestational age, a due date, and — if you know your LMP — whether ACOG guidelines suggest redating your pregnancy.
Medically reviewed by Charlotte Rose, RN · Reviewed for accuracy: July 2026 · Based on the Hadlock (1984) formulas and ACOG Committee Opinion 700
Enter scan details
Enter any measurements shown on your report — you do not need all four. More measurements generally improve accuracy, especially in the second trimester.
We will compare LMP-based dating with your ultrasound dating using the ACOG redating thresholds below.
Your results
Enter a scan date and at least one measurement, then tap “Calculate gestational age.”
How ultrasound estimates gestational age
Once a pregnancy moves past the first trimester, sonographers switch from measuring the whole embryo (crown-rump length) to measuring specific fetal structures: the width of the head (BPD), the circumference of the head (HC), the circumference of the belly (AC), and the length of the thigh bone (FL). Each of these grows at a predictable, well-studied rate, so its size can be run backward through a regression formula to estimate gestational age. The formulas used here come from Hadlock et al. (1984), the same equations that built the growth curves used in ultrasound machines and radiology departments worldwide.
Quick answer
If your report lists BPD, HC, AC, and FL together, this ultrasound gestational age calculator combines all four into a single composite estimate, which is generally more accurate than any one measurement alone. If you only have one or two measurements, it estimates gestational age from those and averages them, then projects your estimated due date as 280 days minus your gestational age in days, counted forward from your scan date.
Which measurement matters most, by trimester
| Gestational age | Best-performing measurement | Typical accuracy (±) |
|---|---|---|
| Up to 13 weeks 6 days | Crown-rump length (CRL) | About 5 to 7 days |
| 14 to 21 weeks | BPD and HC (composite biometry) | About 7 to 10 days |
| 22 to 27 weeks | Composite of BPD, HC, AC, FL | About 10 to 14 days |
| 28 weeks and beyond | Composite of BPD, HC, AC, FL | About 21 to 30 days |
Notice the pattern: the earlier the scan, the tighter the accuracy window. That is why ACOG considers a first-trimester ultrasound the single most reliable way to date a pregnancy, and why a third-trimester scan is used to assess growth far more often than to set a due date. If your scan falls before 14 weeks and reports a crown-rump length instead of BPD/HC/AC/FL, use our Crown-Rump Length Calculator instead — the formulas here are built for second- and third-trimester biometry.
The Hadlock formulas behind this calculator
For each measurement (entered in centimeters), the calculator solves the matching Hadlock regression for menstrual age in weeks:
| Measurement | Formula (GA in weeks, measurement in cm) |
|---|---|
| BPD | GA = 9.54 + 1.482·BPD + 0.1676·BPD² |
| HC | GA = 8.96 + 0.540·HC + 0.0003·HC³ |
| AC | GA = 8.14 + 0.753·AC + 0.0036·AC² |
| FL | GA = 10.35 + 2.460·FL + 0.170·FL² |
| BPD + HC + AC + FL (composite) | GA = 10.85 + 0.060·(HC·FL) + 0.670·BPD + 0.168·AC |
When you enter every one of BPD, HC, AC, and FL, the calculator leads with the composite equation, since combining parameters improves accuracy over any single measurement. When only some are entered, it estimates gestational age from each one individually and averages the results — a method described in obstetric ultrasound literature as improving reliability by letting several parameters point toward the same answer.
ACOG guidelines for redating a pregnancy
If you already had a due date from your last menstrual period and your ultrasound comes back on a different day, that does not automatically mean the due date should change. ACOG Committee Opinion 700 sets specific discrepancy thresholds by gestational age, above which redating to the ultrasound is recommended:
| Gestational age at scan | Redate if LMP and ultrasound differ by more than |
|---|---|
| Up to 8 weeks 6 days | 5 days |
| 9 weeks 0 days – 13 weeks 6 days | 7 days |
| 14 weeks 0 days – 15 weeks 6 days | 7 days |
| 16 weeks 0 days – 21 weeks 6 days | 10 days |
| 22 weeks 0 days – 27 weeks 6 days | 14 days |
| 28 weeks 0 days and beyond | 21 days |
If you tick “I also know my last menstrual period” and enter that date, the calculator applies this exact table automatically and tells you whether ACOG guidance supports keeping your LMP-based due date or shifting to the ultrasound-based one. Once an early, accurate scan has set a due date, ACOG advises against changing it again on later scans — differences later in pregnancy usually reflect fetal size, not dating.
A worked example
Suppose a scan at what is presumed to be 20 weeks by LMP reports a BPD of 4.7 cm, HC of 17.6 cm, AC of 15.4 cm, and FL of 3.3 cm. Run through the composite formula, that biometry lands around 20 weeks 3 days — only a few days apart, well inside the 10-day threshold for the 16–21-week window, so ACOG guidance would keep the original LMP-based due date. If that same scan had come back closer to 18 weeks 2 days, the nearly two-week gap would cross the threshold, and the due date would typically be revised to match the ultrasound.
Related maternal health calculators
This tool is part of the Maternal Health Calculators hub. These companion tools cover cycle tracking, ultrasound dating, and maternity planning — each answers a different question, so pick whichever fits where you are.
Frequently asked questions
Each measurement is run through its own Hadlock (1984) regression formula to produce a gestational age estimate. When all four are available, the calculator uses a combined equation that weighs them together, which research shows is generally more accurate than relying on a single measurement.
The Hadlock formulas are regression equations published by Hadlock and colleagues in 1984 that convert fetal biometric measurements (BPD, HC, AC, FL) into an estimated menstrual age. They remain the most widely used method for ultrasound dating and fetal weight estimation in obstetric practice today.
Third-trimester biometry is the least precise dating method, with an accuracy of roughly plus or minus 21 to 30 days. That is why ACOG discourages using a first ultrasound performed this late to significantly change an already-established due date, and why late scans are used mainly to track growth rather than to date the pregnancy.
A small gap is normal and does not require any change. ACOG Committee Opinion 700 sets specific thresholds by gestational age — for example, more than 5 days before 9 weeks or more than 21 days after 28 weeks — above which the due date is typically shifted to match the ultrasound.
Any single fetal measurement can be converted to a gestational age using its own Hadlock formula, so a report with just an FL or just a BPD is still usable. Accuracy improves as more measurements are added, since averaging several parameters reduces the effect of any one unusual reading.
You can, but a first-trimester scan usually reports crown-rump length (CRL) rather than BPD, HC, AC, or FL, and CRL is the more accurate measurement that early. Our Crown-Rump Length Calculator is built specifically for that measurement and gestational age range.
It gives an estimated due date based on the biometry and scan date you enter, but your medical record uses the due date your provider formally established, which may combine ultrasound with LMP dating under ACOG’s redating rules. Treat this tool as an educational cross-check, not a replacement for that record.
Early in pregnancy, all fetuses grow at very similar rates, so size closely tracks age. Later on, individual growth variation increases — some babies are simply larger or smaller than average — so size becomes a less precise stand-in for age, widening the accuracy window.
Yes. Use the mm / cm toggle above the input fields — most ultrasound reports list biometry in millimeters, and the calculator converts automatically before running the formulas.
Methodology
This calculator converts each entered measurement to centimeters, then applies the corresponding Hadlock (1984) regression formula for BPD, HC, AC, and FL. When all four measurements are present, it uses the four-parameter composite equation (GA = 10.85 + 0.060·(HC·FL) + 0.670·BPD + 0.168·AC), which combines the parameters directly. When only some measurements are available, it computes each one’s individual estimate and averages them. The estimated due date is calculated as the scan date plus 280 days minus the gestational age in days at the time of the scan. When a last menstrual period date is supplied, the calculator computes gestational age by LMP as of the scan date, compares it with the ultrasound-based estimate, and applies the gestational-age-specific discrepancy thresholds from ACOG Committee Opinion 700, Table 1, to indicate whether redating is typically supported. All results are population-based estimates for education and planning, not a clinical dating determination, and are not a substitute for the due date documented by a qualified provider.
Charlotte Rose, RN
A professional nurse turned SEO strategist and web designer with 10+ years of experience in the online health field. Charlotte reviewed this calculator’s biometry formulas and educational content for clinical accuracy and clarity.
References & trusted sources
- ACOG — Committee Opinion No. 700: Methods for Estimating the Due Date
- Hadlock FP, et al. — Estimating fetal age: computer-assisted analysis of multiple fetal growth parameters. Radiology, 1984
- NICHD — Fetal Growth Studies
- American College of Radiology / AIUM — Obstetric Ultrasound Practice Parameters
- MedlinePlus — Fetal biometry and ultrasound dating
This calculator provides educational estimates based on the measurements you enter and is not a diagnostic tool. Individual pregnancies vary, and dating accuracy depends on scan quality, fetal position, and gestational age. If you are pregnant or planning care, please rely on the due date and gestational age documented by your healthcare provider rather than this or any online tool. Last reviewed & updated: July 2026.