Due Date from the Last Menstrual Period
Calculate the date 40 weeks after the last menstrual period begins. ACOG explains this dating convention, while a clinician may establish a different due date using ultrasound or embryo-transfer information.
- Runs in your browser
- Nothing is uploaded
- No signup; device limits apply
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How to use this tool
Select the first day of the last period, not its final day. This basic form has no cycle-length or embryo-transfer input, so it cannot adjust for irregular cycles or assisted reproduction.
Results and examples
The displayed date is the input plus 280 days. It does not promise the actual birth date; use the dating established by your clinician for clinical decisions and appointment planning.
Worked example
An LMP beginning on 15 January 2026 produces an estimated date of 22 October 2026.
Limitations
- There is no ultrasound or cycle-length adjustment.
- The date alone cannot assess fetal health or urgency of care.
Fields
| Field | Guidance and constraints |
|---|---|
| First day of last menstrual period | Check the unit and format displayed beside this field. |
Data handling
Processing stays in the current browser tab; the site does not upload tool input for the calculation itself.
Sources
Frequently asked questions
Should I enter the last day of bleeding?
No. Use the first day of the last menstrual period.
What if my clinician gives another date?
Their dating may incorporate ultrasound and other clinical information.
Related tools
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