How to use the Pregnancy Due Date Calculator
- Choose whether you know the first day of your last period or the conception date.
- Enter the date (and your usual cycle length if not 28 days).
- See your estimated due date, current week and trimester dates.
About this tool
Doctors usually estimate the due date using Naegele’s rule: 280 days (40 weeks) after the first day of the last menstrual period, adjusted if your cycle is longer or shorter than 28 days.
Only about 4–5% of babies arrive exactly on their due date — most are born within two weeks either side. An early ultrasound gives the most accurate dating, so your doctor or midwife may adjust this date.
Frequently asked questions
How accurate is a due date calculator?
It gives a good estimate, but only a small share of babies are born on the exact date. Ultrasound dating is more precise.
How many weeks is a full-term pregnancy?
About 40 weeks from the last period; 39–40 weeks is considered full term.
Is my data saved?
No, everything is calculated in your browser.