- Estimated due date
- 2027-04-07
- Gestational age on selected date
- 14 weeks, 2 days
- Estimated conception
- 2026-07-15
How it works
Revised dating shifts the whole estimate
Moving an entered due date three days later also moves the reverse-calculated conception estimate three days later. That is a consequence of the dating input, not independent evidence about when fertilization happened.
Why conception timing is an estimate
Working backward from a due date or another pregnancy date gives a modeled conception timeline, not proof of a particular event. Different dating inputs and their uncertainty can shift the estimate. Select the mode corresponding to the information you actually know. This calculator cannot establish paternity or identify an exact fertilization date from an approximate pregnancy timeline.
Last menstrual period: formula and assumptions
Estimated due date = LMP + 280 days + (cycle length−28).
Gestational age is conventionally counted from the last menstrual period. Ultrasound and clinical assessment can revise the estimate.
Conception date: formula and assumptions
Estimated due date = conception + 266 days.
A conception date is generally an estimate unless linked to assisted reproduction.
IVF transfer: formula and assumptions
Estimated due date = transfer date + 266 − embryo age days.
Confirm assisted-reproduction dates with your treating clinic.
Ultrasound dating: formula and assumptions
Estimated due date = scan date + (280−gestational days).
Use the gestational age supplied in the clinical ultrasound report.
From a known due date: formula and assumptions
Estimated conception=due date−266 days; conventional gestational start=due date−280 days.
This reverses a 40-week dating convention. It does not establish an exact conception date.
Worked example
Using the Last menstrual period example:
- First day of last menstrual period
- 2026-07-01
- Typical cycle length (days)
- 28
- Calculate gestational age on
- 2026-10-09
Estimated due date = LMP + 280 days + (cycle length−28).
Example result
Estimated due date: 2027-04-07
Gestational age on selected date: 14 weeks, 2 days
Estimated conception: 2026-07-15
These are illustrative inputs. Change the values to explore a different scenario.
What to keep in mind
Dates are estimates, not a prediction of the exact day of delivery.
Formula references
References explain the underlying methods; they do not imply endorsement or individual professional review of Finflo.
Common questions
Does subtracting 266 days from a due date give an exact conception date?
No. It reverses a standard dating convention and produces an estimate, not proof of the exact date of conception.
Can this establish which sexual encounter led to pregnancy?
No. Dating uncertainty and the biological timing involved prevent this calendar estimate from establishing a specific encounter or parentage.
Prepared by Finflo. See our methodology and editorial policy. Read the relevant professional-advice limitations.