One calculator for every situation
There are several ways to calculate a due date, and each works best with different information. Rather than making you choose, this calculator lets you enter everything you know and then applies the order of reliability that clinicians use:
- IVF transfer date — the embryo’s age is known exactly.
- Ultrasound — especially in the first trimester; used instead of your period date if the two differ by more than the allowed margin for that stage.
- Last menstrual period — adjusted for your cycle length.
- Conception date — when nothing else is available.
It shows the due date each piece of information gives, which one it chose, and a plain-language reason.
Why different methods disagree
A period-based date assumes ovulation around day 14 of a 28-day cycle. If you ovulated later — common with longer or irregular cycles — the period date runs ahead of reality, and an early scan will show a smaller baby than expected. That is not a problem with the baby; it simply means the pregnancy started later than the standard assumption. The redating rule is explained in detail on the EDD Calculator Pregnancy page.
What to do with your result
Use the chosen date for planning, and share it with your care team at your next appointment. If they give you a different date, theirs takes priority — update it here using the Due Date Calculator by Due Date so all your other dates line up.