How is my due date calculated?
Your due date is typically estimated based on the first day of your last menstrual period (LMP), assuming a 28-day menstrual cycle with ovulation around day 14. Using this starting point, pregnancy is calculated as 40 weeks from your LMP—even though conception usually occurs about two weeks later.
Alternatively, if you know your exact conception date, your due date is set at 38 weeks from that day. For IVF pregnancies, the calculation is even more precise:
3-day embryo transfer: Add 263 days (38 weeks) to the transfer date.
5-day embryo transfer: Add 261 days (37 weeks and 6 days) to the transfer date.
Ultrasound measurements (like crown-rump length in early pregnancy) can also refine the due date, especially if your cycles are irregular or LMP is uncertain. Healthcare providers often use the earliest ultrasound as the most accurate reference.
While these methods provide a reliable estimate, only about 5% of babies are born on their exact due date—most arrive within a two-week window before or after. Always consult your doctor to confirm your personalized timeline.