Reverse Due Date Calculator
Work backwards from a target due date to the conception window and LMP date that would lead to it.
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What this tool does
Work backwards from a target due date to the conception window and LMP date that would lead to it. Planning arithmetic on average gestation lengths. Natural conception timing cannot be scheduled to a specific day; this is a rough planning guide only.
How to use the Reverse Due Date Calculator
- Enter or select target due date (yyyy-mm-dd).
- Read the calculated result; change any measurement to compare alternatives.
Formula
conception ≈ target due date − 266 days; LMP ≈ target due date − 280 days; conception window shown as ±3 days
- due
- Target due date (YYYY-MM-DD)
Planning arithmetic on average gestation lengths. Natural conception timing cannot be scheduled to a specific day; this is a rough planning guide only.
Worked example
For reverse due date calculator, the following measurements illustrate the exact method: Target due date (YYYY-MM-DD): 2027-09-01.
Inputs
- Target due date (YYYY-MM-DD)2027-09-01
Result
- Estimated conception date2026-12-09
- Likely conception window2026-12-06 through 2026-12-12
- Corresponding LMP date2026-11-25
Results explained
- Estimated conception date
- Estimated conception date from the formula above. Planning arithmetic on average gestation lengths. Natural conception timing cannot be scheduled to a specific day; this is a rough planning guide only.
- Likely conception window
- Likely conception window from the formula above. Planning arithmetic on average gestation lengths. Natural conception timing cannot be scheduled to a specific day; this is a rough planning guide only.
- Corresponding LMP date
- Corresponding LMP date from the formula above. Planning arithmetic on average gestation lengths. Natural conception timing cannot be scheduled to a specific day; this is a rough planning guide only.
Frequently asked questions
It subtracts the average conception-to-birth span (266 days) from your target due date to estimate when conception would need to occur, and 280 days for the corresponding LMP.
Not precisely — conception depends on ovulation timing and chance, and due dates themselves are estimates (only about 5% of babies arrive on them). Use this for rough season-level planning.
The tool shows a ±3-day window around the estimated conception date; the fertile window itself spans roughly 5 days before ovulation through the day after.
In a textbook 28-day cycle, ovulation/conception falls about 14 days after the LMP — the convention behind the standard 280-day due-date rule.
Delivery scheduling is a clinical decision made much later based on the pregnancy itself; this planner only does calendar arithmetic.