LMP and ultrasound markers on a 42-week gestational rulerLive current inputs
Health & Fitness
Gestational Age Estimate Calculator
Translate elapsed menstrual days and a dated ultrasound estimate into weeks plus days, display their discrepancy, and preserve the clinician-selected best obstetric estimate rather than silently redating a pregnancy.
Selected gestational age—
Selected age in total days—
Cycle-adjusted LMP estimate—
Advanced ultrasound estimate—
Method discrepancy—
Days to 40w0d reference—
Calendar phase—
Dating boundary—
DATING RECONCILIATION
LMP and ultrasound markers on a 42-week gestational ruler
Two method markers remain visible while the documented method controls the selected-age marker; trimester bands provide calendar context without changing the date.
DATING REGISTER
Method arithmetic and discrepancy
Each dating route shows its inputs, correction, advanced age, and role in the documented estimate.
| Method | Starting evidence | Adjustment | Current total days | Weeks + days | Documentation role |
|---|
DATING INPUT
Use dates and reports that can be verified
- Count from the first day of the last menstrual period, not the last day.
- Enter cycle length only when it is reasonably known and usually regular.
- Copy weeks and days directly from the ultrasound report.
- Advance the scan age by exact elapsed days.
- Use the method documented by the obstetric clinician.
BEST OBSTETRIC ESTIMATE
Discrepancy is evidence for review, not permission to redraft the due date
ACOG recommends determining and documenting gestational age and EDD once reliable LMP and/or early ultrasound evidence is available. Later changes should be uncommon and documented.
This calculator preserves both estimates and the selected method. It does not implement every gestation-specific redating threshold or ART dating rule.
DATING METHOD
Advance dated evidence without automatically replacing the EDD
The LMP route adjusts the conventional 28-day assumption by entered cycle length. The ultrasound route advances the gestational age recorded on the scan by elapsed calendar days. The selected method is explicit.
Detailed calculation process and general formulas
GA_LMP = days_since_LMP - (cycle_length - 28)GA_scan0 = 7 x scan_weeks + scan_daysGA_scan_now = GA_scan0 + days_since_scanDelta = GA_scan_now - GA_LMPdays_to_40 = 280 - GA_selectedSymbols, meanings, and units
- GA
- gestational agedays
- cycle_length
- entered usual menstrual cycle lengthdays
- GA_scan0
- age documented at ultrasounddays
- Delta
- ultrasound minus LMP discrepancydays
- GA_selected
- age from the explicitly selected documented methoddays
DATING QUALITY
Three facts that deserve separate lines
Age, discrepancy, and dating source answer different questions.
Selected age
—The explicit method determines the headline gestational age.
Method discrepancy
—The difference remains visible instead of being averaged away.
Source boundary
—The page reports which route supplied the selected value.
Decision takeaway: Bring both dates and the earliest reliable ultrasound report to the obstetric clinician when the estimates disagree.
Applied decisions
Dating situations that require different interpretation
Regular cycle with close early scan
Cycle-adjusted LMP and early ultrasound differ by only a few days.
What the result clarifies: The ruler shows agreement without implying that both estimates should be averaged.
Uncertain LMP with documented ultrasound
The menstrual date is uncertain and the clinician documented the ultrasound estimate.
What the result clarifies: Selecting ultrasound preserves the documented dating source while retaining the LMP comparison.
Worked default scenario
Current-input substitution and reconciliation
Method references
Evidence used to frame this specific model
Scope and limitations
Calendar estimate only. It does not establish or revise an estimated due date, assess fetal growth, determine viability, or replace obstetric care. ART pregnancies and uncertain or conflicting dates require clinician-specific dating.
Gestational Age Estimate Calculator | Compare LMP and Ultrasound Dating FAQ
Why not average LMP and ultrasound age?
Clinical dating follows an evidence hierarchy and documented criteria, not a simple arithmetic average.
Does cycle adjustment replace ultrasound?
No. It only adjusts the conventional LMP assumption and may still be unreliable.
What if the pregnancy resulted from IVF?
ART-derived dating uses transfer date and embryo age; use the clinician-documented EDD.
Can later ultrasound change the due date?
Redating depends on gestational timing, discrepancy, earlier evidence, and clinical judgment.