Last Menstrual Period (LMP) Due Date Calculator
Enter the first day of your last menstrual period to calculate your estimated due date — with optional cycle length adjustment for maximum accuracy.
LMP Due Date Calculator
About the Last Menstrual Period Method
The Last Menstrual Period (LMP) method is the standard clinical approach for determining a pregnancy due date. Developed by 19th-century obstetrician Franz Karl Naegele, it remains the most commonly used technique worldwide. The formula is simple: take the first day of your last period and add 280 days (40 weeks) to get your Estimated Due Date (EDD).
Why 280 Days?
- The average menstrual cycle is 28 days
- Ovulation typically occurs around day 14
- Fertilization occurs shortly after ovulation
- Fetal development takes approximately 266 days from conception
- 266 days (from conception) + 14 days (from LMP to ovulation) = 280 days
Cycle Length Matters
Not everyone has a 28-day cycle. Cycle lengths between 21 and 45 days are perfectly normal. Our calculator lets you enter your average cycle length and automatically adjusts the due date: if your cycle is longer than 28 days, ovulation (and conception) happened later, so your EDD is later. If your cycle is shorter, your EDD moves earlier. This is called the modified Naegele’s Rule.
When to Confirm with Ultrasound
ACOG recommends confirming your LMP-based due date with an ultrasound at 8–14 weeks. If the ultrasound measurement differs by more than 7 days in the first trimester, the ultrasound date is preferred. That said, your LMP date is an excellent estimate before your first appointment.
Frequently Asked Questions
Common questions answered by our medical team
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