Conception Calculator
Estimate Conception returns one date: 14 days after a last period, or 266 days before a due date. It is not a fertile window.
How to use
- Set Based on to Last period (LMP) or Due date. The date label switches between LMP date and Due date.
- Enter that date.
- Click Estimate Conception. The result is one calendar date, and the detail line spells the same approximate conception date in words.
When it helps
Last period mode adds 14 days to the LMP, the fixed middle of an assumed 28-day cycle. Due date mode subtracts 266 days, which is 280 minus 14, so both modes point at the same kind of assumed day. You still receive one date, not a range of fertile days.
For the current week and trimester from an LMP, use the pregnancy calculator. To move a due date when the cycle is not 28 days, use the due date calculator. This form has no cycle-length field.
Limits
The date is a rough estimate, not a diagnosis of when conception occurred. It is one approximate date, not a fertile window, and not proof of the day fertilization happened.
A blank date shows Enter a date. The tool does not ask for ovulation tests, temperature, or intercourse dates. It still adds 14 days, or subtracts 266, even when a real cycle is much longer or shorter than 28 days.
Questions
Why 14 days after the LMP?
The page uses a fixed offset. Day 14 is the assumption built into the button, not a reading from your cycle.
Why subtract 266 days from a due date?
280 minus 14 is 266. Due date mode walks back to that same assumed conception day.
Can it list fertile days?
No. Estimate Conception returns a single date. There is no start day and no end day of a window.