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Obstetric Imaging
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Pregnancy Dating (Ultrasound Biometry)

Calculate gestational age and estimated delivery date from ultrasound measurements. Critical for African maternal health.

This calculator estimates gestational age and the estimated date of delivery from ultrasound fetal biometry. In the first trimester, crown-rump length (CRL) provides the most accurate dating; from the second trimester onward, a composite of biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL) is used. Accurate dating underpins screening timing, growth assessment, and safe management of post-term and preterm pregnancies, and is particularly valuable where reliable menstrual dates are unavailable.

🧮 Interactive Calculator

📐 Formula

First trimester: CRL-based (Hadlock). Second/third: BPD, HC, AC, FL composite. Most accurate <14 weeks.

Reference: Hadlock formulas (1984-1991)

When to Use

  • Establishing gestational age when the last menstrual period is unknown, uncertain, or irregular
  • Confirming or reassigning dates in early pregnancy when ultrasound and menstrual dating disagree beyond accepted limits
  • Calculating the estimated date of delivery to guide screening, surveillance, and delivery planning
  • Assessing fetal size and screening for growth restriction or macrosomia in later pregnancy
  • Dating pregnancies at first presentation for antenatal care, especially in low-resource or late-booking settings

How It Works

  • First-trimester dating uses CRL measured in millimetres, most accurate before 14 weeks, when biological size variation is minimal (Hadlock/Robinson regression equations)
  • Second- and third-trimester dating uses a composite of BPD, HC, AC, and FL entered in millimetres and combined via published regression models (commonly Hadlock)
  • Each biometric parameter is converted to an implied gestational age; the composite estimate averages them, weighted to reduce the influence of any single distorted measurement
  • Estimated date of delivery is derived by adding the remaining weeks to reach 40 completed weeks (280 days) from the ultrasound-assigned age
  • The method assumes accurate, standardised image planes and calliper placement, and normal fetal growth; abnormal growth degrades dating accuracy in later pregnancy
  • Dating accuracy is highest early (CRL ±5-7 days) and progressively worsens with advancing gestation (third trimester ±2-3 weeks)

Interpreting the Result

  • First-trimester CRL dating is accurate to approximately ±5-7 days and should be preferred whenever available
  • Redate the pregnancy if ultrasound differs from LMP by more than about 5-7 days in the first trimester, or more than 7-10 days early in the second trimester (per ACOG/AIUM/ISUOG thresholds)
  • Second-trimester composite dating carries ±7-10 day accuracy; third-trimester dating is least reliable (±2-3 weeks) and should not override an earlier scan
  • Once an early accurate scan has assigned dates, later scans are used to assess growth, not to reassign the estimated date of delivery
  • A large discrepancy between measured biometry and established dates in the third trimester suggests a growth abnormality rather than a dating error
  • Estimated date of delivery is a probabilistic midpoint; spontaneous delivery normally occurs across a range around it, and term is defined as 37-42 weeks

Limitations & Pitfalls

  • Dating accuracy declines with advancing gestation; a third-trimester scan should never be used to redate a pregnancy already dated by an early scan
  • Abnormal fetal growth (IUGR, macrosomia), skeletal dysplasias, and structural anomalies distort individual biometric parameters
  • Measurement quality depends on correct anatomical planes, calliper placement, and operator skill; suboptimal images introduce systematic error
  • Reference equations (Hadlock, INTERGROWTH-21st, WHO) differ, and using population-mismatched charts can bias estimates
  • Multiple gestations, oligohydramnios, fibroids, and maternal habitus can impair image acquisition and measurement reliability
  • The estimated date of delivery is a statistical estimate, not a fixed date; management decisions should integrate clinical context
  • Very early gestational sac or yolk-sac-only findings without a measurable CRL cannot be reliably dated by this method

References

  • Hadlock FP et al. Estimation of fetal weight and gestational age with the use of fetal biometry. Radiology / AJR
  • AIUM-ACR-ACOG-SMFM-SRU Practice Parameter for the Performance of Standard Diagnostic Obstetric Ultrasound Examinations
  • ACOG Committee Opinion No. 700: Methods for Estimating the Due Date
  • ISUOG Practice Guidelines: performance of first-trimester and mid-trimester fetal ultrasound scans
  • Papageorghiou AT et al. International standards for fetal growth based on serial ultrasound measurements: the INTERGROWTH-21st Project. Lancet 2014

⚕️ For clinical decision support only. Always verify calculations and apply clinical judgment.