Observed ultrasound ages versus calendar-advanced ageLive current inputs
Health & Fitness
Gestational Age Trend Calculator
Compare three documented ultrasound age estimates with elapsed calendar time, quantify dating residuals, and distinguish serial consistency from fetal-growth assessment.
Scan 2 dating residual—
Scan 3 dating residual—
Largest absolute residual—
Residual direction—
Total calendar advance—
Documented-age advance—
Serial consistency flag—
Clinical boundary—
SERIAL DATING VIEW
Observed ultrasound ages versus calendar-advanced age
The first scan anchors an expected calendar line; later scan labels sit above or below it with explicit day residuals instead of being blended into a new due date.
SCAN LEDGER
Expected and documented age at each scan
The register preserves calendar interval, expected age, documented age, and residual separately.
| Scan | Elapsed calendar days | Expected age | Documented age | Residual | Interpretation limit |
|---|
SERIAL INPUT
Copy report dates and ages exactly
- Use the gestational age printed on each report.
- Calculate elapsed calendar days from scan dates.
- Do not substitute fetal weight percentile or a biometric measurement.
- Preserve the earliest reliable dating record.
- Review discrepancies with the obstetric clinician.
DATING VERSUS GROWTH
A later biometric age is not a second clock
Ultrasound dating accuracy generally decreases with advancing gestation. Once an EDD is established, later biometric estimates are interpreted in clinical context rather than repeatedly resetting the calendar.
This trend view exposes arithmetic consistency only. It cannot diagnose growth restriction, macrosomia, dating error, or measurement-quality problems.
SERIAL CONSISTENCY
Advance the first documented age by real calendar time
The first scan is used only as an arithmetic anchor. Later documented ages are compared with the age expected from elapsed days. The residual is not a growth measurement.
Detailed calculation process and general formulas
G1 = 7w1 + d1G2_expected = G1 + gap12e2 = G2_documented - G2_expectedG3_expected = G1 + gap12 + gap23e3 = G3_documented - G3_expectedSymbols, meanings, and units
- G
- documented ultrasound gestational-age estimatedays
- gap
- elapsed calendar interval between scansdays
- G_expected
- calendar-advanced age from scan 1days
- e
- documented minus expected dating residualdays
- w,d
- whole weeks and additional daysweeks, days
RESIDUAL REVIEW
Magnitude, direction, and timing all matter
The same day residual can mean different things at different gestational ages.
Second-scan residual
—Shows departure from the first calendar interval.
Third-scan residual
—Shows whether the direction persists or reverses.
Largest departure
—Keeps the most prominent difference visible for review.
Decision takeaway: Use the chart to ask how the documented EDD was established and how later measurements should be interpreted.
Applied decisions
Serial patterns that should not be overinterpreted
Small stable residuals
Both later scans remain close to calendar progression.
What the result clarifies: The arithmetic is consistent, but this alone does not assess fetal wellbeing.
Residual grows later
The third documented biometric age falls further from the calendar line.
What the result clarifies: The result supports clinician review without automatically redating the pregnancy.
Worked default scenario
Current-input substitution and reconciliation
Method references
Evidence used to frame this specific model
Scope and limitations
This calculator compares documented gestational ages with elapsed days. It does not estimate fetal growth, revise the EDD, interpret ultrasound quality, or diagnose any pregnancy condition.
Gestational Age Trend Calculator | Serial Dating Consistency FAQ
Should later ultrasound age replace the first estimate?
Not automatically. Established dating follows clinical criteria and documented evidence.
Is a negative residual fetal growth restriction?
No. Dating residuals are not a diagnosis and do not replace biometric growth assessment.
Why anchor the first scan?
It provides a transparent arithmetic reference; the clinician determines which evidence is authoritative.
What residual is acceptable?
The calculator sets no universal clinical threshold because accuracy and interpretation depend on scan timing and context.