A due date calculator estimates the expected delivery date by adding 280 days (40 weeks) to the first day of the last menstrual period, then adjusting for cycle length.
About this calculator
A due date calculator estimates the expected delivery date by adding 280 days (40 weeks) to the first day of the last menstrual period, then adjusting for cycle length. Women with cycles longer or shorter than 28 days have correspondingly later or earlier ovulation, shifting the due date.
Early ultrasound (before 14 weeks) is the most accurate method for confirming gestational age and due date, especially for women with irregular cycles.
Common uses
- Estimate due date with a known cycle length
- Adjust due date for cycles longer than 28 days
- Plan maternity leave and hospital pre-registration
- Determine the approximate conception date
Frequently asked questions
How does cycle length affect the due date?
Ovulation typically occurs 14 days before the next period. For a 35-day cycle, ovulation is on day 21 instead of day 14. This means conception and due date are both shifted 7 days later compared to a 28-day cycle. The calculator adjusts for this automatically.