Due Date by Ultrasound — GA & CRL
Estimated due date from an ultrasound scan. Enter gestational age (weeks + days) or crown-rump length (CRL) in mm. EDD, GA, trimester and milestones.
CalcVita. (2026). Due Date by Ultrasound — GA & CRL. CalcVita. Retrieved June 23, 2026, from https://calcvita.com/en/calculators/ultrasound-due-date
How to use the Ultrasound Due Date calculator
Choose your input method: Gestational Age (GA) mode if your ultrasound report shows the number of weeks and days, or CRL mode if your report gives the crown-rump length in mm. Enter the date of the scan and the measurement, then click Calculate.
Gestational Age (GA) input mode
In GA mode, enter the gestational age shown on your ultrasound report as weeks + days (e.g., "12 weeks 3 days"). The EDD is calculated as: scan date + (280 − total GA days). This is the standard ACOG method for adjusting the due date based on a dating scan.
Crown-Rump Length (CRL) input mode
In CRL mode, enter the crown-rump length from your ultrasound in mm (valid range: 10–84 mm, corresponding to roughly 7–14 weeks). The Robinson & Fleming formula converts CRL to GA: GA (weeks) = CRL/10 + 6.5. This is then used to calculate the EDD. CRL dating is most reliable in the first trimester.
EDD formula (ACOG/ISUOG)
EDD = scan date + (280 − GA at scan in days). LMP-equivalent = EDD − 280 days. Current GA = GA at scan + days elapsed since scan. Trimester 1: < 14 weeks; Trimester 2: 14–27 weeks; Trimester 3: ≥ 28 weeks.
Scientific References
- Robinson HP, Fleming JE (1975). A critical evaluation of sonar crown-rump length measurements. Br J Obstet Gynaecol 82(9):702–710. PMID 1181692
- ACOG Committee Opinion No. 700 (2017). Methods for Estimating the Due Date. Obstet Gynecol 129(5):e150–e154.
- Salomon LJ et al.; ISUOG (2019). ISUOG Practice Guidelines: performance of first-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol 53(1):1–11.
- Hadlock FP et al. (1982). Fetal crown-rump length: reevaluation of relation to menstrual age (5–18 weeks). Radiology 182(2):501–505.
- How does ultrasound pregnancy dating work?
- Ultrasound dating works by measuring the size of the baby and comparing it to established growth norms to estimate gestational age. In the first trimester, the crown-rump length (CRL) is the most accurate measure — it can date a pregnancy within 5–7 days. In the second trimester, multiple measurements (head circumference, femur length, abdominal circumference) are used, with accuracy of ±1–2 weeks. The estimated due date (EDD) is calculated as: EDD = scan date + (280 − gestational age at scan in days).
- What is CRL and when is it measured?
- Crown-rump length (CRL) is the distance from the top of the baby's head (crown) to its bottom (rump), measured in millimetres. It is the primary dating method in the first trimester, typically between 6 and 14 weeks of gestation (CRL 10–84 mm). The Robinson & Fleming (1975) formula used in this calculator converts CRL to gestational age: GA (weeks) ≈ CRL/10 + 6.5. A dating scan at 11–13+6 weeks with CRL measurement is recommended by ISUOG to establish or confirm the due date.
- Should I use my LMP or the ultrasound for my due date?
- ACOG (2017) and ISUOG recommend using ultrasound dating when the difference between the LMP-based EDD and the ultrasound-based EDD exceeds a threshold: 5 days in the first trimester (≤13+6 weeks), 7 days at 14–15+6 weeks, 10 days at 16–21+6 weeks, 14 days at 22–27+6 weeks, and 21 days at ≥28 weeks. When cycles are irregular or the LMP is uncertain, ultrasound dating is preferred even earlier. This calculator always uses the ultrasound measurement as the reference.
- How accurate is first-trimester ultrasound dating?
- First-trimester CRL dating (6–13+6 weeks) is the most accurate method, with accuracy within ±5–7 days (95% confidence). This is significantly better than LMP-based dating, which assumes a regular 28-day cycle with ovulation at day 14. Second-trimester biometry is accurate to ±1–2 weeks, and third-trimester measurements are the least accurate (±3–4 weeks). For this reason, the first-trimester dating scan is the gold standard for establishing an EDD.
- What are the key pregnancy milestones shown in this calculator?
- This calculator shows 7 key milestones: Conception (~2 weeks after LMP), End of First Trimester (14 weeks), Anatomy Scan (20 weeks, when structural abnormalities are screened), Start of Third Trimester (28 weeks), Term (37 weeks, when the baby is considered early term), Estimated Due Date (40 weeks), and Post-term (42 weeks). These dates are approximate and calculated from the LMP-equivalent derived from your ultrasound.
- What is an LMP-equivalent date?
- The LMP-equivalent is a calculated reference date that corresponds to the first day of the last menstrual period if the pregnancy had the exact gestational age shown at the scan. It is derived by subtracting 280 days from the estimated due date (EDD − 280 days). The LMP-equivalent is useful for establishing gestational age for clinical records even when the actual LMP is unknown, irregular, or inconsistent with ultrasound measurements.
Suggested article
Ultrasound Pregnancy Dating: CRL, Gestational Age, and How Your Due Date is Calculated
How does a dating scan turn a CRL measurement or gestational age into an estimated due date? This guide explains the Robinson & Fleming formula, ACOG dating guidelines, and how to read your pregnancy milestones.
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