Due Date Calculator from Last Menstrual Period (LMP)

How to Calculate Your Due Date from LMP

The Last Menstrual Period (LMP) method is the most universally used approach for calculating a pregnancy due date. First formalized by German obstetrician Franz Karl Naegele in the 19th century, this technique is still taught in medical schools and used in clinics around the world. To calculate your due date from LMP, simply identify the first day of your last period and add 280 days (40 weeks).

Naegele’s Rule Step by Step

Example: LMP = March 1, 2024 → Add 1 year = March 1, 2025 → Subtract 3 months = December 1, 2024 → Add 7 days = December 8, 2024. Our calculator does all of this instantly, plus adjusts for your specific cycle length.

Cycle Length Adjustment

Standard Naegele’s Rule assumes a 28-day cycle with ovulation at day 14. However, cycle lengths between 21 and 45 days are normal. Women with shorter cycles ovulate earlier, while women with longer cycles ovulate later. Our calculator adjusts for this: if your cycle is 35 days, we add an extra 7 days to your due date estimate. If your cycle is 21 days, we subtract 7 days. This is the most accurate way to calculate due date from LMP without an ultrasound.

LMP vs Ultrasound Dating

ACOG (American College of Obstetricians and Gynecologists) recommends that ultrasound dating at 8–14 weeks be used to confirm the LMP-estimated due date. If the ultrasound measurement differs by more than 7 days from the LMP date (in the first trimester), the ultrasound date is generally preferred. Our last menstrual period calculator gives you the best starting estimate before your first appointment.