How this instrument works
This calculator estimates an estimated date of delivery (EDD) from a known conception date, rather than from the last menstrual period. That distinction matters for anyone who genuinely knows when conception happened — most commonly from an IVF embryo transfer date, or from careful fertility tracking that pins down ovulation directly. Enter the number of days that have passed since conception, and it returns the days remaining to full term.
Full term counted from conception runs about 266 days — 38 weeks — 14 days shorter than the 280-day, 40-week figure this site's LMP-based Due Date calculator uses. The gap is not a discrepancy; it is definitional. LMP-based dating starts the clock at the first day of the last period, which sits roughly two weeks before typical ovulation and conception, so it counts 14 extra days that this calculator does not need, because it already starts from conception itself.
Both routes describe the same underlying pregnancy length — full term is full term either way — they simply start the clock at different points. The American College of Obstetricians and Gynecologists directs that when conception timing is reliably known, as with assisted reproductive technology, that conception-based age should be used to assign the due date rather than LMP-based dating. As with any due-date estimate, treat the result as an average: real deliveries vary by roughly two weeks in either direction.
- Count the number of days that have passed since the known conception date.
- Enter that figure in Days Since Conception — a day count, not a calendar date field.
- Read Days Until Due — the days remaining to the 266-day estimate of full term.
- If only the last menstrual period is known rather than a conception date, use this site's LMP-based Due Date calculator instead, which runs on the 280-day convention.
Worked example — 76 days since conception
Seventy-six days have passed since the known conception date. Subtract that from the 266-day constant: 266 − 76 = 190 days remaining — the same 190-day figure this site's LMP-based example reaches from 90 days since LMP, because 76 days after conception is 90 days after the LMP that preceded it by about two weeks. Same pregnancy, same remaining time, two different starting points.
The edges follow the same subtraction. On the day of conception itself, day 0, the full 266 days are still ahead. At day 266, the countdown reaches zero and the estimate has arrived — though, as with any due-date estimate, real deliveries commonly land anywhere from about two weeks earlier to two weeks later.
Questions
How is this different from a due date calculated from my last period?
The math differs only in its starting point and constant. This calculator counts forward from a known conception date using 266 days (38 weeks) of full-term gestation. This site's other Due Date calculator counts forward from the first day of the last menstrual period using 280 days (40 weeks), because LMP sits roughly 14 days before typical conception. Both describe the same length of pregnancy; they simply start the clock differently.
What counts as a known conception date?
The clearest case is an IVF embryo transfer, where the embryo's age and the transfer date are recorded precisely. Careful fertility tracking — basal body temperature, ovulation predictor kits, or cervical mucus charting pinned to a specific day — can also establish it. Without one of these, most people are better served by this site's LMP-based Due Date calculator, since a remembered last-period date is usually more reliable than a guessed conception date.
Why is the conception-based estimate 266 days instead of 280?
Because it starts the clock 14 days later. The 280-day, 40-week figure runs from the first day of the last menstrual period, which sits roughly two weeks before ovulation and conception in a typical cycle. Start counting from conception itself instead, and the same full-term pregnancy takes about 266 days, 38 weeks. Neither number is more correct — they measure the same event from two different reference points.
Is a conception-based due date more accurate than an LMP-based one?
It can be, but only when the conception date is genuinely well established, such as with an IVF transfer. ACOG guidance directs that assisted-reproduction dating be used over LMP dating when it is available, since it removes the guesswork around ovulation timing. For a conception date estimated from ordinary tracking rather than a clinical procedure, the honest uncertainty is closer to LMP-based dating.
Does this calculator accept a calendar date for conception?
No — it takes a day count. Enter the number of days that have already passed since the known conception date rather than the date itself; the underlying rule, 266 days of full-term gestation from conception, stays the same either way.
How much can the real delivery date vary from this estimate?
By about two weeks in either direction, same as any due-date estimate. Full-term delivery is commonly described as a range rather than a single day, and this calculator's 266-day figure marks the middle of that range rather than a deadline. Treat it as a planning anchor, not a prediction of the exact day.
References
- ACOG Committee Opinion — Methods for Estimating the Due Date
- ACOG FAQ — When Pregnancy Goes Past Your Due Date
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.