Pregnancy Due Date Calculator

Calculating Your Estimated Due Date

The standard method for calculating a due date is Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This convention was established by German obstetrician Franz Naegele in the 19th century and remains the most widely used clinical method today, despite the fact that only 4–5% of babies are born on their exact estimated due date.

When Is Your Due Date Most Likely to Shift?

• Irregular cycles: Naegele's Rule assumes a 28-day cycle with ovulation on day 14. Longer cycles shift the EDD later; shorter cycles shift it earlier.
• First-trimester ultrasound: If the ultrasound-measured gestational age differs from the LMP-calculated age by more than 5–7 days (before 10 weeks) or 10–14 days (after 14 weeks), the ultrasound date takes precedence.
• IVF pregnancies: Dating is highly accurate — a Day 5 transfer uses Transfer Date + 261 days for EDD.

Pregnancy Timeline Overview