How do you calculate the last menstrual period (LMP) in a patient?

Calculating Last Menstrual Period (LMP)

The most accurate method to calculate last menstrual period (LMP) is to record the first day of the last menstrual bleeding as the LMP date, which serves as a critical reference point for determining gestational age in pregnancy.

Definition and Importance

LMP refers to the first day of the most recent menstrual period and is a fundamental measurement used in obstetrics to:

• Calculate gestational age
• Determine estimated due date (EDD)
• Guide timing of prenatal screening tests
• Inform clinical decision-making throughout pregnancy

Methods for Determining LMP

Direct Patient Recall

• Ask the patient to identify the first day of their most recent menstrual bleeding
• Research shows that:
  • 56% of women accurately recall their LMP date
  • 74% recall within 1 day of accuracy
  • 81% recall within 2 days 1
• Accuracy decreases with longer recall periods, with women tending to overestimate the time since LMP when recall duration exceeds 3 weeks 1

Calendar-Based Methods

• Providing patients with a calendar to mark their menstrual periods prospectively improves accuracy 2
• Home calendar methods have shown good concordance with ultrasound measurements in validation studies 2

Documentation Considerations

• Record LMP as a specific date (MM/DD/YYYY)
• Express gestational age in completed weeks and days (e.g., 15 weeks and 5 days) rather than rounded weeks 3
• Document the method used to determine LMP (patient recall, calendar record, etc.)

Validation and Correction of LMP

Ultrasound Validation

• First-trimester ultrasound (especially crown-rump length measurement) provides the most accurate validation of gestational age 3
• When significant discrepancy exists between LMP and ultrasound dating:
  • If gestational age changes by 2 or more weeks after ultrasound, the test results and dating must be reinterpreted 3
  • Ultrasound dating is generally preferred when discrepancies occur 3

Factors Affecting LMP Accuracy

• Maternal characteristics associated with greater LMP-ultrasound discrepancies:
  • Younger maternal age
  • Non-Hispanic Black and Hispanic ethnicity
  • Non-optimal body weight
  • Mothers of low-birthweight infants 4
• Biological variation in the interval from LMP to actual conception (standard deviation of approximately 7 days) 5

Clinical Applications

For Pregnancy Dating

• LMP is used to calculate gestational age by counting from the first day of the last period
• Estimated due date (EDD) is traditionally calculated as LMP + 280 days (Nägele's rule)
• For accurate pregnancy dating, ultrasound confirmation is recommended, especially for:
  • Women with irregular cycles
  • Women who cannot recall their LMP
  • Cases where LMP and clinical examination are discrepant 3

For Prenatal Screening

• Accurate LMP is critical for timing of screening tests:
  • First trimester screening: 11-13 weeks
  • Second trimester screening: 15-20 weeks 3
• Inaccurate LMP dating may lead to inappropriate timing of screening tests and false results

Common Pitfalls and Solutions

Potential Errors

• Mistaking spotting or irregular bleeding for a true period
• Recall bias, especially with longer time intervals
• Biological variation in ovulation timing relative to menstruation
• Irregular menstrual cycles affecting accuracy

Solutions

• Use standardized questioning techniques for determining LMP 6
• Implement prospective recording methods when possible
• Confirm gestational age with ultrasound when:
  • LMP is uncertain
  • Menstrual cycles are irregular
  • Clinical size differs from expected for dates
  • Prenatal screening decisions depend on precise dating

Algorithm for LMP Determination

1. Ask patient: "What was the first day of your last menstrual period?"
2. Document the reported date and level of certainty
3. If uncertain, use auxiliary questions:
   • "Do you track your periods on a calendar or app?"
   • "Was this period normal in timing and flow?"
4. For pregnancy dating:
   • If LMP is certain and cycles are regular: Use LMP for initial dating
   • If LMP is uncertain or cycles irregular: Prioritize ultrasound dating
   • If discrepancy >7-10 days between LMP and ultrasound: Use ultrasound dating

By following this systematic approach to determining LMP, clinicians can optimize the accuracy of gestational age assessment, which is crucial for appropriate prenatal care and clinical decision-making.