Calculating Gestational Age from Last Menstrual Period
Gestational age is calculated by counting the number of completed weeks and days from the first day of your last menstrual period to today's date. 1
Standard Calculation Method
- Add 280 days (40 weeks) to the first day of your LMP to determine your estimated due date (EDC). 1
- Count forward from the first day of your LMP to today to determine your current gestational age, expressed as weeks plus days (e.g., "12 weeks 3 days"). 1
- This method assumes a 28-day menstrual cycle with ovulation occurring around day 14, though actual cycle lengths vary. 2
Key Assumptions and Limitations
LMP-based dating assumes regular menstrual cycles and accurate recall of the LMP date, but these assumptions are frequently violated in clinical practice. 3, 4
- Women with menstrual cycles longer than 28 days will have their gestational age overestimated by LMP dating, leading to falsely elevated rates of post-term delivery (up to 14.6% vs. 5-10% actual). 2
- Women with shorter cycles will have their gestational age underestimated. 2
- LMP dating is unreliable when the date is uncertain, when there has been recent hormonal contraceptive use, during lactation, or when bleeding unrelated to menses occurred. 3, 5
When LMP Dating Should NOT Be Used
Never use LMP dating for pregnancies conceived through assisted reproductive technology (ART)—the due date must be calculated from the embryo transfer date and embryo developmental stage. 1
Verification with Ultrasound
Early ultrasound (ideally 11-13 weeks) provides gestational age estimates accurate to within 7 days and should be used to verify or replace LMP dating when available. 6
- First-trimester ultrasound using crown-rump length (CRL) is more accurate than LMP dating, with LMP estimates averaging 0.8 days longer than ultrasound estimates. 4
- Ultrasound correctly classifies term, preterm, and post-term deliveries more accurately than LMP (84%, 68%, and 86% vs. 79%, 61%, and 55% respectively). 7
- When both reliable LMP and early ultrasound are available, ultrasound dating is preferred for establishing the official estimated due date. 8, 3
Clinical Pitfalls to Avoid
- Do not round gestational age to the nearest week—always report as completed weeks plus days (e.g., "28 weeks 3 days" not "28 weeks"). 1
- LMP dating becomes increasingly inaccurate in women with irregular cycles, young women, non-Hispanic Black and Hispanic women, and women of non-optimal body weight. 4
- The traditional Ogino-Knaus correction (subtracting or adding days based on cycle length) tends to overcorrect and is not recommended. 2