What is the recommended method for pregnancy dating, using the first day of the last menstrual period (LMP) or the last day of the last menstrual period?

Pregnancy Dating: First Day of Last Menstrual Period is the Recommended Method

Pregnancy dating should be calculated using the first day of the last menstrual period (LMP), not the last day of the last menstrual period. 1 This is the standard method endorsed by major medical societies for determining gestational age in clinical practice.

Evidence-Based Rationale

The Society of Radiologists in Ultrasound (SRU) consensus guidelines clearly state that gestational age (GA) is calculated using the first day of the last menstrual period and/or an early-dating ultrasound examination 1. This approach is consistently reinforced across multiple guidelines:

• The duration of pregnancy is conventionally reported as gestational age and refers to completed weeks from the first day of the last menstrual period 1
• The American College of Radiology and ACOG recommend reporting gestational age as the number of weeks plus the number of days (e.g., 12 weeks 3 days) 2

Clinical Application of First Day LMP Dating

Using the first day of LMP provides several advantages:

• Establishes a standardized starting point for pregnancy dating
• Allows for consistent comparison across clinical settings
• Provides a reference point even before conception occurs (as ovulation typically occurs around day 14 of the menstrual cycle)

Accuracy Considerations

While first day of LMP is the standard method, it's important to note:

• LMP-based estimates average 0.8 days longer than ultrasound estimates 3
• LMP may classify more births as post-term than ultrasound (4.0% vs. 0.7%) 3
• Discrepancies between LMP and ultrasound dating can indicate increased risk of pregnancy loss 4

Ultrasound Dating as Complementary Method

Early ultrasound dating serves as an important complement to LMP dating:

• Crown-rump length measurement between 7-14 weeks provides accuracy of ±7 days 2
• Ultrasound dating is superior to LMP in predicting delivery date by at least 1.7 days 5
• When ultrasound is used instead of certain LMP, the number of post-term pregnancies decreases from 10.3% to 2.7% 5

Common Pitfalls to Avoid

1. Using last day of LMP: This is not the standard method and will result in incorrect gestational age calculations.

2. Relying solely on LMP without ultrasound verification: LMP has limitations including:

   • Reporting problems such as uncertainty regarding the LMP date
   • Delayed ovulation can result in invalid estimates 6
   • Low sensitivity (29.0%) in identifying late-term neonates 7

3. Ignoring significant discrepancies: When ultrasound-based dating lags LMP-based dating by more than 3 days, there's a fivefold increased risk of pregnancy loss 4

Algorithm for Pregnancy Dating

1. Primary method: Calculate gestational age from the first day of the last menstrual period
2. Verification: Perform early ultrasound dating (ideally between 7-14 weeks)
3. Reconciliation: If discrepancy exists between LMP and ultrasound:
   • For discrepancies ≤7 days: LMP dating can be maintained
   • For discrepancies >7 days: Use ultrasound dating as it's more accurate
4. Documentation: Report gestational age as number of weeks plus days (e.g., 12 weeks 3 days)

In summary, the first day of the last menstrual period is the standard starting point for pregnancy dating, ideally verified with early ultrasound for optimal accuracy in clinical practice.