Standard pregnancy dating estimates due date at 266 days after conception (or 280 days after last menstrual period).
How to use this due date to conception calculator
If you already know your estimated due date, this calculator helps you work backward to find an estimated conception date. Enter your due date, keep the default 266-day interval (or adjust it if instructed by your clinician), and click calculate.
You will get:
- An estimated conception date
- A likely conception window (about 5 days)
- An estimated last menstrual period (LMP) date
How the calculation works
The formula is simple:
Estimated conception date = Due date − 266 days
This is based on standard obstetric dating where pregnancy duration is usually measured as 280 days from LMP, and ovulation/conception often occurs about 14 days after LMP in a typical cycle.
Example
If your due date is December 10, 2026, the calculator estimates conception around March 19, 2026, with a likely fertile window around that date.
Why conception date is only an estimate
Even with ultrasound and cycle tracking, conception usually cannot be pinned to a single exact day. Several factors can shift the estimate:
- Ovulation can occur earlier or later than expected
- Sperm can survive up to 5 days in reproductive fluid
- Implantation timing varies from person to person
- Due date itself is an estimate, not a guarantee
Conception date vs. LMP date
| Term | What it means | Typical timing |
|---|---|---|
| Last Menstrual Period (LMP) | First day of your last period used for standard pregnancy dating | About 280 days before due date |
| Conception date | Approximate date fertilization occurred | About 266 days before due date |
| Ovulation date | When an egg is released | Usually near conception, often same day |
When this calculator is most useful
- You have a due date from your provider and want a rough conception timeline
- You are organizing personal records or pregnancy milestones
- You want to compare dates with cycle tracking notes
Special cases to keep in mind
IVF pregnancies
For IVF, conception/fertilization dating may be known more precisely based on retrieval and transfer dates. In those cases, clinic dates are more accurate than a generalized reverse due-date estimate.
Irregular cycles
If your cycles are irregular, LMP-based due date estimates may have larger uncertainty. First-trimester ultrasound is often the best dating reference.
Preterm or post-term births
Actual birth date can happen before or after estimated due date. That does not necessarily change the original conception estimate—it just reflects normal variability in labor timing.
Quick FAQ
Can this tool tell me the exact day I conceived?
No. It gives a best estimate and a likely window.
Is due date always 40 weeks?
Clinically, due date is set at 40 weeks from LMP (280 days), which is about 38 weeks from conception (266 days).
Should I trust this over my doctor’s dates?
No. Use this as educational support only. Follow your clinician’s dating, especially if ultrasound-based.
Important medical note
This calculator is for informational purposes and does not diagnose, treat, or replace professional medical advice. For personalized pregnancy dating and care decisions, consult your obstetric provider or midwife.