Calculator
Estimated due date calculator
Choose the correct reference: first day of the last period, known fertilisation or embryo transfer. A regular cycle can be adjusted from 28 days; IVF dating uses embryo age at transfer. A clinically established date takes precedence over self-calculation.
Calculator
Enter data
Method, matching reference date and regular-cycle length for LMP mode. Fields marked with an asterisk are required.
How to use the calculator
- Choose a method and verify what the date means. Do not mix transfer and fertilisation dates.
- Confirm the result in prenatal care; do not use this calculator to change dating established through clinical assessment.
Calculation details
Formula and calculation method
LMP + 280 days + (cycle length − 28). Known fertilisation + 266 days. Day-3 transfer + 263 days; day-5 transfer + 261 days.
Worked example
An LMP of 1 January 2026 with a regular 28-day cycle gives 8 October 2026. A day-5 embryo transfer on that same date adds 261 days, not 280.
Limitations
- Cycle adjustment is an approximation for regular 21–35-day cycles, not ovulation confirmation. Other embryo ages and ultrasound measurements are not supported.
- Intercourse date is not a known fertilisation date. The estimate does not guarantee a birthday or replace prenatal care.
Sources
Engine version: 2.0 (2026-09-22) · Last reviewed: 22 September 2026
Frequently asked questions
Why is the offset different after IVF?
Embryo age at transfer is already known: 3 or 5 days are subtracted from 266. This method does not count from intercourse.
Which date should I use if clinical dating differs?
Discuss the difference with your maternity care team. The calculator does not make re-dating decisions or interpret measurements.