Verified Mathematical Model & Formulation
EDD (Naegele's Rule) = LMP Date + 1 Year - 3 Months + 7 Days (Adjusted for Cycle Length: + (Cycle - 28) Days) Gestational Age = Days Elapsed since LMP / 7 Weeks
Calculates the Estimated Date of Delivery (EDD) based on Naegele's clinical obstetric standard (40 weeks / 280 days from the first day of the last menstrual period, calibrated to a woman's average menstrual cycle length).
Configurable Parameters & Standard Units
- Dating Basis (1=Last Menstrual Period LMP, 2=Conception Date): (Default: 1 count)
- LMP / Conception Month (1 - 12): (Default: 1 months)
- LMP / Conception Day (1 - 31): (Default: 15 days)
- Year (e.g. 2026): (Default: 2026 years)
- Average Menstrual Cycle Length (Days): (Default: 28 days)
Standard Worked Example & Practical Verification
Scenario: A mother whose last menstrual period began on January 15 with a regular 28-day cycle.
Practical Takeaway: First trimester concludes at week 13, second trimester spans weeks 14-27, and third trimester begins at week 28.
Frequently Asked Questions
How accurate is an ultrasound compared to the LMP due date calculator?
First-trimester crown-rump length (CRL) ultrasound measurements between 8 and 13 weeks are the clinical gold standard for dating, typically accurate within 5 to 7 days.
What if my menstrual cycle is longer or shorter than 28 days?
Our calculator automatically applies an offset: cycles longer than 28 days delay expected ovulation and push the due date back, while shorter cycles move the due date earlier.
When does each trimester begin and end?
The 1st trimester lasts from conception through week 13 (day 91). The 2nd trimester spans week 14 through week 27 (day 189). The 3rd trimester lasts from week 28 through delivery (week 40+).