Calculating Gestational Age from LMP of January 31,2025
To calculate the age of gestation (AOG) by last menstrual period when LMP is January 31,2025, count the number of completed weeks and days from January 31,2025 to the current date, expressing the result as weeks and days (e.g., 8 weeks 3 days). 1
How to Calculate Gestational Age
Gestational age is calculated from the first day of the last menstrual period and is reported as the number of weeks and number of days. 1
Start counting from January 31,2025 (day 1 of the pregnancy) and count forward to today's date to determine the current gestational age. 1
Express the result in the format of weeks and days (such as 10 weeks 2 days) rather than rounding to whole weeks, as precision is critical for clinical decision-making. 2
Important Clinical Context
LMP-based dating should be documented in the medical record, but when first-trimester ultrasound with crown-rump length (CRL) measurement becomes available, the ultrasound-based date should be used for all clinical decisions if there is a discrepancy of ≥5 days. 2
First-trimester ultrasound (CRL measurement between 8-13 weeks) is the gold standard for pregnancy dating with accuracy within ±5-7 days, making it superior to LMP-based calculations. 2, 3
The first trimester is defined as gestational age ≤13 weeks 6 days, which is relevant for determining appropriate timing of screening tests and clinical management. 1
Common Pitfalls to Avoid
Never average LMP and ultrasound dates - always use the ultrasound date when available in the first trimester and when there is a discrepancy. 2
LMP-based dating assumes a 28-day cycle with ovulation on day 14, which may not be accurate for all patients, leading to potential dating errors. 4, 5
Accurate dating is crucial for appropriate timing of Down syndrome and neural tube defect screening, as serum screening results are highly dependent on precise gestational age. 2