Pregnancy Due Date Calculator – Naegele's Rule
Estimate your baby's due date from the first day of your last menstrual period. This calculator uses Naegele's rule (LMP + 280 days), the standard method used by obstetricians. See your estimated due date, current weeks pregnant and trimester. For conception planning, pair with our ovulation calculator.
Pregnancy Due Date Calculator
Estimate your baby's due date using Naegele's rule. Enter the first day of your last menstrual period (LMP). Assumes a 28-day cycle with ovulation around day 14.
How Naegele's rule works
Franz Naegele (early 1800s) observed that full-term pregnancy averages about 40 weeks from the last menstrual period:
Due date = LMP + 280 days (or LMP + 40 weeks)
Alternative shortcut: subtract 3 months from LMP, add 1 year and 7 days. The rule assumes ovulation on day 14 of a 28-day cycle. Ultrasound in the first trimester can provide a more accurate estimate when cycle length varies.
Planning pregnancy and understanding the due date
The estimated due date (EDD) helps you and your healthcare provider plan prenatal visits, screenings and prepare for birth. Most pregnancies last 37–42 weeks; only a small percentage deliver on the exact date. First-trimester ultrasound can adjust the date if your cycle is irregular.
For conception planning, use the ovulation calculator to estimate your fertile window. The due date calculator complements that by projecting when your baby might arrive based on LMP. This tool is for educational use; always follow your healthcare provider's guidance.