Ultrasound Due Date Accuracy
Ultrasound Due Date Accuracy
First-trimester ultrasound is the gold standard for gestational dating because embryonic growth is highly uniform. Crown-rump length measured before 14 weeks predicts gestational age within ±5 to 7 days, outperforms LMP when cycles are irregular or uncertain, and helps determine whether to change the due date.
ACOG Redating Thresholds
The LMP-based due date is changed only if the ultrasound-estimated gestational age differs from it by more than the following:
|
Gestational Age at Ultrasound |
Redate if Discrepancy Is Greater Than |
|---|---|
|
≤ 8 6/7 weeks |
More than 5 days |
|
9 0/7 to 15 6/7 weeks |
More than 7 days |
|
16 0/7 to 21 6/7 weeks |
More than 10 days |
ACOG additionally recommends redating when the discrepancy exceeds 14 days at 22 0/7–27 6/7 weeks and 21 days at 28 0/7 weeks and beyond, though later ultrasounds are poor tools for dating (see below).
Additional rules
- If LMP is unknown or uncertain, the best available ultrasound sets the due date.
- If ultrasound and LMP agree within the thresholds, the LMP date is kept.
- Once the due date is established by a first-trimester scan, it is not changed by later scans.
- In IVF, dating is based on embryo age and transfer date, not ultrasound.
Key Biometric Measurements
Trimester 1: Crown-Rump Length (CRL)
CRL is the length of the embryo from the top of the head to the bottom of the torso. It is measured from about 6 weeks to 13 6/7 weeks and is the most accurate dating measurement because early growth is so consistent between embryos. Before about 6 weeks, mean sac diameter and yolk sac are used if the embryo is not yet visible.
Trimesters 2 and 3: composite biometry
- Biparietal diameter (BPD): the width of the skull at the level of the thalami.
- Head circumference (HC): often paired with BPD.
- Abdominal circumference (AC): the most sensitive indicator of growth and the most affected by nutrition.
- Femur length (FL): the length of the thigh bone.
Equations combine these measurements into a single estimated gestational age.
Ultrasound Accuracy by Timing
| Scan Timing (Weeks) | Measurement Used | Margin of Error (± Days) | Primary Clinical Objective |
|---|---|---|---|
| 6–8 weeks | CRL (or mean sac diameter if no embryo seen) | ±5 | Confirm intrauterine location, cardiac activity, and establish the most accurate due date |
| 9–13 weeks | CRL | ±7 | Dating when the early scan was missed; nuchal translucency screening |
| 18–20 weeks (Anatomy Scan) | BPD, HC, AC, FL | ±10 (threshold); true variability is greater | Structural anatomy survey, not dating |
| 32+ weeks (Growth Scan) | BPD, HC, AC, FL | ±21 (threshold; individual error is often wider) | Estimate fetal weight, growth, and fluid; not dating |
Why Later Ultrasounds Are Unreliable for Redating
Biometric variability between normal fetuses increases with gestational age. By the late second and third trimester, genetics, maternal size, and placental function influence size much more than age:
Genetic variation
Parental height and ethnicity affect expected size. A big baby is not necessarily an older baby.
Fetal growth restriction (FGR)
A small fetus due to placental insufficiency would appear “younger” than true gestational age. Redating would hide a diagnosis that needs surveillance.
Macrosomia
Maternal diabetes or constitutional factors can enlarge a fetus beyond its gestational age, and redating forward would incorrectly label the baby as postterm.
For these reasons, size discrepancies in later pregnancy are treated as growth findings, not dating errors.
Frequently Asked Questions
Can a baby’s due date change after a 20-week anatomy scan?
Rarely. If no earlier ultrasound exists and the LMP is unknown, the due date can be set from the 20-week scan, but it is a less precise estimate (±10 days or more). If dating was already established by LMP or an earlier scan, a difference under 10 days at 16 0/7–21 6/7 weeks does not change it. Size differences are usually reported as growth percentiles.
What happens if my ultrasound date differs by 4 days from my LMP?
In most cases, nothing changes. At less than 9 weeks, redating happens only when the difference exceeds 5 days. At 9 0/7 to 15 6/7 weeks, the threshold is more than 7 days. A 4-day difference is within normal biological variation, so your LMP-based due date is kept. If your LMP is uncertain or your cycles are irregular, your clinician may use the ultrasound date instead.
Is an ultrasound or my period calculator more accurate?
Dating by a first-trimester ultrasound is more accurate than LMP when cycles are irregular, the LMP is uncertain, or ovulation was likely not on day 14. With a certain LMP and regular 28-day cycles, LMP and early ultrasound agree closely. Calculators based on LMP carry the same limits as LMP; see the Irregular Periods Due Date Calculator and Conception Date vs. LMP.
How accurate is a 6-week transvaginal ultrasound?
Very accurate for dating, within about ±5 days using CRL. At 6 weeks, a transvaginal scan also commonly shows cardiac activity, and the embryo is small enough that a 1–2 mm difference in CRL translates into a few days. Very early scans (before the embryo is visible) are less precise for dating and are sometimes repeated in 1–2 weeks.