Converts elapsed days since the last menstrual period into completed weeks and remaining days. This guide explains the formula, use and limitations.
Overview
Converts elapsed days since the last menstrual period into completed weeks and remaining days.
Clinical Significance
Gestational age based on LMP assumes reliable dates and may be revised by early ultrasound according to guidance.
When to Use
- Use when the calculation is relevant to the clinical question.
- Confirm that the population and units match the validated method.
- Use current and reliable measurements.
How It Is Calculated
- Completed weeks = floor(days ÷ 7); remaining days = days modulo 7.
Interpretation
- Gestational age based on LMP assumes reliable dates and may be revised by early ultrasound according to guidance.
- Interpret alongside the full clinical picture and current guidance.
Worked Example
Enter the required measurements in the stated units. MedicalC then applies: Completed weeks = floor(days ÷ 7); remaining days = days modulo 7.
Patient Considerations
- Consider age, body size, pregnancy, organ function and treatment effects where relevant.
- Repeat the calculation when inputs or clinical status materially change.
Limitations
- Accuracy depends on the quality and units of the entered data.
- The equation may not be validated in every patient population.
- This tool does not replace clinical judgement or specialist advice.
Clinical Pearls
- Check units before calculating.
- Document the source and timing of each input.
- Use trends where clinically appropriate.
Common Mistakes
- Mixing SI and conventional units.
- Using non-contemporaneous measurements.
- Treating the output as a diagnosis or prescription.
Evidence Base
The implemented calculation follows the published or guideline-based relationship: Completed weeks = floor(days ÷ 7); remaining days = days modulo 7.
Frequently Asked Questions
What does the Gestational Age from LMP Interval calculator do?
Converts elapsed days since the last menstrual period into completed weeks and remaining days.
Can the result be used alone?
No. Use it as decision support with clinical assessment and current guidance.
Why might the result be misleading?
Incorrect units, poor measurement quality or use outside the validated population can produce misleading results.
Should it be recalculated?
Recalculate when relevant measurements or the clinical condition change.
References
- ACOG Committee Opinion: Methods for Estimating the Due Date. — Primary publication, guideline or established source supporting the calculation.
Reviewed by: MedicalC Clinical Editorial Team
Last reviewed: July 2026