How is a due date calculated?
The last menstrual period method estimates a due date by adding 280 days to the first day of the last period, with a cycle-length adjustment when needed.
Due date from conception
A conception-based estimate adds 266 days to the conception date, because this method starts around two weeks after the LMP dating method.
Due date from IVF
IVF dating adjusts for embryo age. A 3-day transfer adds 263 days, while a 5-day transfer adds 261 days.
Limitations
This is an estimate. Ultrasound and clinical dating may differ, and pregnancy dates should be confirmed by a qualified healthcare professional.