Pregnancy Ultrasound Dating Accuracy: Understanding Margins of Error

Ultrasound gestational age (GA) estimation has well-defined margins of error that vary by trimester. A discrepancy of up to 1 week (or more) is common and expected.

Accuracy by Trimester (Singleton Pregnancies)

Trimester	GA Range	Measurement	Accuracy (± days)
First	≤ 13 6/7 wks	CRL (crown-rump length)	± 3–5 days (best < 12 wks)
Second	14–27 6/7 wks	BPD, HC, AC, FL (biometry)	± 7–10 days
Third	≥ 28 wks	Biometry (multiple)	± 14–21 days

Sources of Variability

• Biological variation: Normal fetal size range at given GA (genetics, placental function, maternal factors).
• Measurement error: Inter-observer (sonographer), intra-observer, machine calibration, fetal position.
• Dating assumption: LMP-based GA assumes 28-day cycle, day-14 ovulation. Real cycles vary.
• Conception timing: Sperm survival (5 days), ovulation variability → GA uncertainty even with known LMP.

Clinical Protocol for Discrepancies

• First trimester: If US GA differs from LMP by > 5 days (≤ 8 6/7 wks) or > 7 days (9–13 6/7 wks) → re-date by US.
• Second trimester: If US GA differs from LMP by > 10 days → re-date by US.
• Third trimester: US not used for dating; growth assessment only.
• Suboptimal images: Repeat in 1–2 weeks (do not re-date on poor study).

What "Wrong by a Week" Means

• At 8 weeks: ± 5 days = normal. A "week off" is within expected error.
• At 20 weeks: ± 7–10 days = normal. A "week off" is within expected error.
• At 32 weeks: ± 2–3 weeks = normal. A "week off" is well within error.

When to Worry

• Serial scans show growth restriction (AC < 10th %ile, crossing percentiles ↓).
• Discrepancy > expected margin + abnormal anatomy / Doppler / fluid.
• Known dates (ART, ovulation tracking) conflict significantly with US.

Action: Discuss with your OB/midwife. They integrate LMP, all US data, clinical exam, and hCG trends to assign the best obstetric estimate (BOE) — the official EDD used for management.