Free Pregnancy Tool
Pregnancy Due Date
Estimate your due date from your last period, your conception date or your IVF transfer date.
General information only, not medical advice. Ask your doctor or midwife.
How this works
Formula: Due date = first day of last period + 280 days + (cycle length − 28). From conception: + 266 days. IVF: transfer + 266 − embryo age.
- Naegele's rule adds 280 days (40 weeks) to the first day of your last period. That assumes a 28-day cycle.
- If your cycle is longer or shorter, ovulation is later or earlier, so the calculator moves the due date by the difference.
- From a known conception date it adds 266 days (38 weeks).
- For IVF it adds 266 days to the transfer date and takes away the embryo's age: 3 days for a day-3 embryo, 5 days for a day-5 embryo.
Everything is worked out in your browser. Belly Weekly does not send or store your answers.
Questions
How many babies arrive on their due date?
Only a small share. The due date is a guide; most babies arrive in the weeks around it.
Will my doctor change my due date?
Possibly. An early ultrasound can measure the baby, and your doctor may adjust your due date if it differs from your period-based date.
What if I don't know my last period?
Use your conception date if you know it, or ask your doctor about a dating ultrasound.