Due Date by Ultrasound — GA & CRL
Work out gestational age from an ultrasound: enter crown-rump length (CRL) in mm — or weeks + days directly. Get current GA, due date and trimester.
CalcVita. (2026). Due Date by Ultrasound — GA & CRL. CalcVita. Retrieved November 6, 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 (days) = 8.052 × √CRL + 23.73. 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 1182090.
- ACOG Committee Opinion No. 700 (2017). Methods for Estimating the Due Date. Obstet Gynecol 129(5):e150–e154. PMID 28426621.
- Salomon LJ et al.; ISUOG (2013). ISUOG Practice Guidelines: performance of first-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol 41(1):102–113. PMID 23280739.
- Hadlock FP et al. (1992). Fetal crown-rump length: reevaluation of relation to menstrual age (5–18 weeks) with high-resolution real-time US. Radiology 182(2):501–505. PMID 1732970.
- 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 an accuracy of about ±7–14 days. 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 main dating measurement in the first trimester, for CRLs of 10–84 mm (about 7 to 14 weeks). This calculator converts CRL to gestational age with the Robinson & Fleming (1975) equation: GA (days) = 8.052 × √CRL (mm) + 23.73. ISUOG recommends a dating scan with CRL measurement at 11–13+6 weeks to establish or confirm the due date.
- Should I use my LMP or the ultrasound for my due date?
- ACOG, AIUM and SMFM (Committee Opinion 700, 2017) recommend replacing the LMP-based due date with the ultrasound one when the difference exceeds: 5 days before 9+0 weeks, 7 days at 9+0–13+6 weeks, 7 days at 14+0–15+6, 10 days at 16+0–21+6, 14 days at 22+0–27+6 and 21 days from 28+0 weeks. When the LMP is unknown or cycles are irregular, the pregnancy is dated by ultrasound, ideally the earliest CRL scan. This calculator always uses the ultrasound measurement as the reference.
- How accurate is first-trimester ultrasound dating?
- First-trimester CRL dating (up to 13+6 weeks) is the most accurate method, with an accuracy of ±5–7 days according to ACOG. This is better than LMP-based dating, which assumes a regular 28-day cycle with ovulation on day 14. Second-trimester biometry is accurate to ±7–10 days up to 21+6 weeks and ±10–14 days from 22+0 to 27+6 weeks; third-trimester measurements are the least accurate (±21–30 days). 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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