Due date calculator from conception date: how it works
By Pregora Editorial Team · Updated 2026-04-28 · 6 min read
If you know your conception date — for example, you tracked ovulation, conceived during a single fertile encounter, or had IVF — you can calculate your pregnancy due date directly without using the last menstrual period (LMP) method. The formula is simple: add 266 days to the conception date.
This guide explains exactly how the conception-to-due-date calculation works, when to use it instead of LMP-based dating, and the small adjustments that improve accuracy.
The 266-day rule
Pregnancy lasts about 266 days from conception. This is sometimes called the "true gestational age" — the time from fertilisation to delivery. Most pregnancy calculators count from LMP and add 280 days because they assume conception happens 14 days after LMP. If you skip that assumption and start from the actual conception date, the math is:
Estimated due date = conception date + 266 days
Our due date calculator handles this automatically when you select "Conception" as the calculation method.
For IVF pregnancies, use our IVF due date calculator instead — it adjusts for embryo age (Day 3, 5, or 6) at transfer.
When to use conception-based dating
Conception-based dating is more accurate than LMP-based dating in these situations:
- You tracked ovulation precisely using OPKs, BBT charting, or fertility apps
- You conceived during a single fertile encounter and the date is known
- You had IVF and know the embryo transfer date (use the IVF tool for best accuracy)
- Your menstrual cycles are very irregular and LMP is meaningless
- You don't remember your LMP but do remember the approximate conception date
Conception date vs ovulation date
Strictly speaking, conception happens when sperm meets egg — which is on or shortly after ovulation. For most calculations, conception date and ovulation date can be used interchangeably. Sperm can survive in the female reproductive tract for up to 5 days, so even if intercourse happened a few days before ovulation, conception still happens at ovulation.
Use our ovulation calculator to find your peak ovulation day if you don't already know it.
How conception-based dating relates to LMP-based dating
The two methods are mathematically related: for a 28-day cycle, conception happens 14 days after LMP. So:
LMP + 280 days = (LMP + 14 days conception) + 266 days
≡ Conception date + 266 days
For longer or shorter cycles, the conception date shifts forward or backward, but the 266-day count from conception stays constant. That's why conception-based dating is more accurate for irregular cycles — it bypasses the cycle length assumption entirely.
Reverse calculation: when did I conceive?
If you already have a confirmed due date and want to know your conception date, the reverse formula is:
Conception date = due date − 266 days
Many people use this to identify the approximate conception window, especially if they had multiple potential conception dates. Our due date calculator's "Reverse" method does this calculation directly when you input your due date.
What about the conception window vs single date?
In reality, conception isn't a single instant — sperm can fertilise the egg up to 5 days after intercourse. If you have a range of possible conception dates rather than a single one, the most likely conception date is on or one day after ovulation. Calculate from that, and your due date estimate will be within ±2-3 days of the most accurate possible value.
Frequently asked questions
Is conception-based due date more accurate than LMP-based?
Yes — if you genuinely know the conception date. For irregular cycles or unknown LMP, conception dating is significantly more accurate. For regular 28-day cycles with known LMP, both methods give nearly identical results.
How can I figure out my conception date if I don't track ovulation?
For a 28-day cycle, conception is roughly 14 days after the first day of your last period. For longer or shorter cycles, conception is approximately (cycle length − 14) days after LMP. An early ultrasound is the most accurate way to confirm.
Will my conception date match my IVF transfer date?
Not exactly. For IVF, conception (fertilisation) happens at egg retrieval, not at transfer. Day 5 transfer means the embryo is 5 days post-conception when transferred. Use the IVF Due Date Calculator which handles this automatically.
Medical disclaimer: Pregnancy due dates are estimates. Your healthcare provider may revise the date based on early ultrasound measurements. Always consult a qualified healthcare provider about your individual pregnancy.