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Ultrasound Gestational Age & EDD

Robinson & Fleming 1975 (CRL); Hadlock 1984 (biometry)

Estimates gestational age from crown-rump length (Robinson 1975), second-trimester biometry (Hadlock 1984 BPD/HC/AC/FL with composite averaging), or last menstrual period, and derives an estimated date of delivery (Naegele's rule).

Also searched as: obstetrics · crl · bpd · femur length · hadlock · pregnancy dating · pregnancy · dating · obstetric

Crown-rump length (CRL)

Most accurate dating parameter; validated for roughly 6-14 weeks (CRL ~2-84 mm).

mm
Biparietal diameter (BPD)

Hadlock 1984 equation; validated for BPD ~14-102 mm.

mm
Head circumference (HC)

Hadlock 1984 equation; validated for HC ~54-358 mm.

mm
Abdominal circumference (AC)

Hadlock 1984 equation; validated for AC ~49-380 mm.

mm
Femur length (FL)

Hadlock 1984 equation; validated for FL ~6-82 mm.

mm
Days since first day of last menstrual period

Used for LMP-based gestational age and EDD (Naegele's rule, LMP + 280 days; assumes a regular 28-day cycle).

days
Information
Incomplete input
  • Enter at least one of: crown-rump length (CRL), second-trimester biometry (BPD, HC, AC, FL), or the number of days since the last menstrual period.

How this tool works

First-trimester crown-rump length is the most accurate sonographic dating parameter and is converted with the Robinson & Fleming equation as endorsed by BMUS: GA(days) = 8.052 x sqrt(CRL_mm x 1.037) + 23.73. Beyond the first trimester, gestational age is estimated from fetal biometry using the Hadlock 1984 regressions (measurements in cm): BPD: GA(wk) = 9.54 + 1.482(BPD) + 0.1676(BPD^2); HC: GA(wk) = 8.96 + 0.540(HC) + 0.0003(HC^3); AC: GA(wk) = 8.14 + 0.753(AC) + 0.0036(AC^2); FL: GA(wk) = 10.35 + 2.460(FL) + 0.170(FL^2). When two or more biometric parameters are provided, their estimates are averaged into a composite. The estimated date of delivery follows Naegele's rule (LMP + 280 days), and an ultrasound-based EDD is computed analogously from the sonographic age.

References

Implemented from the cited published sources. Educational use only; confirm against current guidelines before clinical use.

Clinical questions that use this

Vetting guides that reach for this tool.

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