Gestational Age From LMP Calculator

Arithmetic obstetric age from independently established LMPdate, not conception verification or confirmation of pregnancy. Manual Gregorian date validation; uncertain cycles, LMPand ultrasound dating must be reviewed clinically.

Description

Gestational age from the last menstrual period counts elapsed calendar days from the recorded menstrual anchor.

Gestational age from the last menstrual period counts elapsed calendar days from the recorded menstrual anchor.^{[1, 2]}

Uncertain dates and cycle variation remain separate from that count.

When to use Gestational Age From LMP Calculator

Use this tool to calculate or review the supplied observations under the stated rules. Establish the observation date, units, measurement context and independently applicable clinical reference or calibrated model.

How to use Gestational Age From LMP Calculator and understand its results

Establish the observation date, units, measurement context and independently applicable clinical reference or calibrated model.
Enter independentlyRecordedLmpDate, asOfDate, using the field meanings and rules below, then run the tool.
Read result.conditionalGestationalDays, result.completedGestationalWeeks, result.additionalGestationalDays. Use the units and sign conventions below.
Correct any reported input issue and run again. Before using a successful result, check the stated units, signs, reference scope and observation coverage.

Input field	Meaning	Value type	Requirement
independentlyRecordedLmpDate	Independently recorded lmp date	string	Required
asOfDate	As of date	string	Required

Parse real Gregorian menstrual-anchor and as-of dates in years 0001–9999. Require elapsed days to be an integer 0–450. Return elapsed days, floor(days/7) completed weeks and days%7 additional days. No uncertainty or cycle adjustment is applied.

gestationalDays=ordinal(asOfDate)−ordinal(lmpDate); completedWeeks=floor(days/7); additionalDays=days%7

Use the matching field units and the exact boundary, ordering and reference rules above.

Output field	Meaning	Value type
result	Structured result; its nested fields below contain the actual calculation or selected reference	object
result.conditionalGestationalDays	Conditional Gestational Days	number
result.completedGestationalWeeks	Completed Gestational Weeks	number
result.additionalGestationalDays	Additional Gestational Days	number

json

{
  "input": {
    "independentlyRecordedLmpDate": "2026-01-01",
    "asOfDate": "2026-02-01"
  },
  "output": {
    "result": {
      "conditionalGestationalDays": 31,
      "completedGestationalWeeks": 4,
      "additionalGestationalDays": 3
    }
  }
}

Limitations and assumptions

Dating evidence, acquisition conditions and applicable clinical references remain independently established. The output describes supplied observations or a conditional model without diagnosing an outcome, choosing a dose or verifying clinical applicability.

Missing required fields, unknown fields, wrong types and violated rules return input issues. Numeric values and outputs must be finite. Extremely large composite outputs are rejected.

Alternative or Complementary methods

For a related step, use Gestational Age From Conception Calculator: Time since conception and obstetric gestational age use different date anchors. For a complementary comparison, use Due Date Naegele Calculator: A cycle-adjusted Naegele calculation shifts a recorded menstrual date by a conventional duration.

References

Gestational Age From LMP Calculator — tool.io
Methods for estimating the due date — American College of Obstetricians and Gynecologists

Similar or alternative tools

Gestational Age From Conception Calculator
Time since conception and obstetric gestational age use different date anchors.
Due Date Naegele Calculator
A cycle-adjusted Naegele calculation shifts a recorded menstrual date by a conventional duration.