Dating Ultrasound Accuracy by Gestational Age
Dating ultrasound is most accurate in the first trimester, specifically between 8-13 weeks (crown-rump length measurement), with accuracy within ±5-7 days, and accuracy progressively decreases as pregnancy advances beyond this window. 1, 2, 3
Optimal Timing for Dating
First Trimester: The Gold Standard Window
Crown-rump length (CRL) measurement between 8-13 weeks provides the highest accuracy for gestational age determination, with precision within 7 days. 2, 3
The earlier the ultrasound assessment in pregnancy, preferably between 10-12 weeks, the better the estimate of gestational age—with up to 92% of deliveries occurring within 37-42 weeks when dated by early ultrasound versus 87% when dated by last menstrual period. 3
Gestational age assessment is most accurate at the first ultrasound imaging and can be accomplished using mean sac diameter (very early) or CRL measurements (once embryo is measurable). 1
Research using assisted reproductive technology pregnancies (where exact conception date is known) confirms first-trimester ultrasound dating is accurate to within approximately ±1.5 days compared to the true gestational age. 4
Second Trimester: Declining Accuracy
In the second trimester, composite ultrasound dating based on multiple measurements (biparietal diameter, head circumference, abdominal circumference, femur length) is accurate to within 10 days—significantly less precise than first trimester CRL. 1, 5
After 24 weeks of gestation, a reliable last menstrual period actually provides better estimates of gestational age than ultrasound measurements. 3
For pregnancies at 14 weeks gestation or later when LMP is uncertain, biparietal diameter (BPD) measurement alone is recommended as it can rule out anencephaly and improve detection of open spina bifida. 1
Clinical Application Algorithm
When to Use Which Dating Method
If ultrasound available before 14 weeks: Use CRL measurement—this supersedes LMP dating regardless of LMP certainty. 2
If first ultrasound at 14-24 weeks: Use composite measurements (BPD, HC, AC, FL) which remain preferable to LMP dating. 1, 5
If first ultrasound after 24 weeks: Consider LMP dating if reliable; ultrasound becomes less accurate for dating purposes. 3
Managing Dating Discrepancies
For any discrepancy ≥5 days in the first trimester, adopt the gestational age determined by CRL measurement—do not average the dates. 2
Document both LMP-based and CRL-based dates in the medical record, but use the CRL-based date for all clinical decision-making including timing of screening tests and subsequent ultrasound examinations. 2
Express adjusted gestational age as weeks and days (e.g., 12 weeks 3 days) or decimal weeks for optimal accuracy. 2
Critical Pitfalls to Avoid
Never average LMP and ultrasound dates—always use the CRL date when available in the first trimester. 2
Do not delay obtaining a dating ultrasound thinking "any time is fine"—accuracy deteriorates significantly after 13-14 weeks, and the clinical utility for dating purposes diminishes progressively. 3, 6
Avoid using the obsolete term "fetal pole" or "embryonic pole"—use "embryo" until 10 weeks 6 days, then "fetus" from 11 weeks 0 days onward. 1, 2
Ensure proper CRL measurement technique: measure the greatest length of the embryo/fetus from crown to rump, excluding limbs and yolk sac. 1
Special Considerations
In multiple gestations, gestational age assessment is most accurate at the first ultrasound imaging using mean sac diameter or CRL measurements, with all attempts made to establish chorionicity and amnionicity at the earliest imaging encounter. 1
Dating formulae originally intended for mid-second trimester use do not necessarily perform well when used earlier in gestation—the systematic and random error must be assessed for the specific gestational age interval. 6
BPD-based dating formulae at 12-14 weeks show smaller random measurement errors (SD 1.86-2.09 days) compared to CRL formulae (SD 2.25-4.86 days), though CRL remains the standard first-trimester dating parameter. 7