Given the patient’s last menstrual period was [DATE] and today is [DATE], what is her gestational age in weeks and days?

Gestational Age Calculation

Based on the last menstrual period (LMP) as [DATE] and today's date as [DATE], calculate gestational age by counting the number of complete weeks and days from the first day of the LMP to today, expressing the result as weeks + days (e.g., 8 weeks 3 days). 1

Standard Calculation Method

• Gestational age is reported as the number of completed weeks and days from the first day of the last menstrual period, which is the standard convention used in obstetrics 1
• Count forward from the first day of LMP to today's date, expressing the result as "X weeks Y days" rather than rounding to the nearest week, as precision is crucial for clinical decision-making 1
• The duration of pregnancy is conventionally 280 days (40 weeks) from the first day of LMP, though this assumes a regular 28-day cycle with ovulation at day 14 2, 3

Important Considerations for Accuracy

Cycle Length Adjustments

• If the patient has irregular cycles or cycles longer/shorter than 28 days, the standard LMP calculation may be inaccurate and should be adjusted or confirmed with ultrasound 4, 3
• For cycles longer than 28 days, ovulation occurs later than day 14, which delays conception and makes LMP-based dating appear more advanced than actual gestational age 4, 3
• Women with long cycles (>35 days) have significantly higher rates of falsely classified post-term pregnancies when using unadjusted LMP dating 3

Validation with Ultrasound

• First-trimester ultrasound with crown-rump length (CRL) measurement between 8-13 weeks provides accuracy within ±5-7 days and is superior to LMP-based dating 4, 2
• When a discrepancy of ≥5 days exists between LMP-based and CRL-based gestational age, adopt the ultrasound-based gestational age for all subsequent clinical decisions 4
• LMP-based estimates average 0.8 days longer than first-trimester ultrasound estimates, with greater discrepancies in young women, non-Hispanic Black and Hispanic women, and women with non-optimal body weight 5

Clinical Application

Immediate Next Steps

• Document the calculated gestational age based on LMP as "X weeks Y days" in the medical record 1
• Schedule or perform first-trimester ultrasound before 14 weeks to confirm dating, as this is the optimal window for accurate CRL measurement 4, 2
• If the patient has not yet established prenatal care, arrange urgent comprehensive initial assessment including dating ultrasound, routine prenatal laboratory testing, and risk stratification 2

Key Timing Milestones

• First-trimester aneuploidy screening should be performed between 11-14 weeks if desired 2
• Anatomy ultrasound is typically performed at 18-22 weeks gestation 2
• The first trimester is defined as gestational age ≤13 weeks 6 days 1
• An embryo is designated as such until 10 weeks 6 days GA; beginning at 11 weeks 0 days GA, the term "fetus" should be used 1

Common Pitfalls to Avoid

• Do not use manual gestational age wheels or calculators, as they have been shown to have significant bias with individual errors up to 4 days; electronic devices are recommended 6
• Do not round gestational age to the nearest week, as precision in days is essential for proper timing of screening tests and clinical interventions 1
• Be aware that LMP-based dating assumes certain knowledge of the first day of LMP and regular 28-day cycles, which may not apply to all patients 3, 5