How do I calculate gestational age based on the last menstrual period (LMP) date?

Calculating Gestational Age from Last Menstrual Period

To calculate gestational age from the LMP date, count the number of completed weeks and days from the first day of the last menstrual period to today's date, expressing the result as weeks and days (e.g., 15 weeks 5 days) or decimal weeks (e.g., 15.7 weeks) rather than rounded weeks. 1

Standard LMP-Based Calculation Method

• Count forward from the first day of the LMP to the current date to determine the number of completed weeks and days of gestation 1
• Express gestational age as weeks and days (15 weeks and 5 days = 15 completed weeks) rather than rounded weeks, as screening performance is improved with this precision 1
• Alternatively, express as decimal weeks (15 weeks and 5 days = 15.7 weeks) for optimal accuracy 1, 2

Important Adjustments for Irregular Cycles

• For women with cycles longer than 28 days, add the difference between their cycle length and 28 days to the standard calculation 3
• For example, with a 35-day cycle, ovulation occurs around day 21 (7 days later than the standard day 14), so add 7 days to the calculated gestational age 3
• This adjustment accounts for delayed ovulation in women with longer menstrual cycles 3

Critical Limitations of LMP Dating

• LMP dating should be replaced by first-trimester ultrasound crown-rump length (CRL) measurement when available, as ultrasound is more accurate (±5-7 days vs. ±14 days for LMP) 2, 4
• LMP-based estimates can be unreliable due to:
  • Uncertainty about the actual LMP date 5
  • Bleeding not associated with true menses 5
  • Delayed ovulation, which occurs even in women with certain LMP dates 5
• When a discrepancy of ≥5 days exists between LMP and first-trimester CRL dating, adopt the CRL-based gestational age for all clinical decision-making 2, 4

Clinical Significance of Accurate Dating

• Accurate gestational age is crucial because maternal serum AFP levels increase by 10-15% per week in the second trimester, making even small dating errors clinically significant 2, 3
• Proper dating affects the timing and interpretation of:
  • Down syndrome screening 2
  • Neural tube defect screening 2
  • All prenatal screening tests 2

Common Pitfalls to Avoid

• Never round gestational age to the nearest week (e.g., don't call 15 weeks 5 days "16 weeks"), as this reduces accuracy 1
• Never average LMP and ultrasound dates - always use the ultrasound date when available in the first trimester 4, 3
• Document both LMP-based and ultrasound-based dates in the medical record, but use the ultrasound-based date for clinical decisions 2

Optimal Dating Strategy

• Obtain first-trimester ultrasound with CRL measurement between 8-13 weeks, which provides accuracy within ±5-7 days and should replace LMP dating 2, 3
• CRL measurement at 11-13 weeks is considered the gold standard, with accuracy as precise as ±1.5 days compared to true gestational age 2
• Second-trimester composite ultrasound (18-22 weeks) is accurate to within 10 days when first-trimester dating was missed 2