Conception Calculator
Working Backward From What You Already Know
Not everyone starts from a last period date. Sometimes the due date is the one fixed point — set by a clinician after a dating scan — and what you actually want to know is roughly when conception happened. Sometimes the baby has already arrived and you're curious, or need the date for a record. This calculator covers all three starting points and reduces each one to the same underlying arithmetic.
The Formulas
From due date: Conception Date = Due Date − 266 days
From birth date: Conception Date = Birth Date − 266 days
266 days is 38 weeks — the length of pregnancy counted from conception rather than from the last period (which runs 40 weeks/280 days because it starts two weeks earlier). The last-period formula instead estimates ovulation directly: cycle length minus the standard 14-day luteal phase.
Why the Three Modes Give Different Precision
| Mode | Assumption | Typical use |
|---|---|---|
| From last period | 14-day luteal phase from your actual cycle length | Early estimate before any scan |
| From due date | Due date is exactly 280 days after an LMP that fit a standard cycle | Working backward from a clinician-confirmed or ultrasound-adjusted due date |
| From birth date | Delivery happened on exactly day 266 from conception | Rough after-the-fact estimate; actual births rarely land on that exact day |
The birth-date mode is explicitly the least precise of the three, since term birth spans weeks 37–42 — a real range of up to five weeks around the 266-day midpoint.
Where This Gets Used
- Paternity or legal timelines — establishing a plausible conception window from a known birth or due date.
- Reconciling a scan-adjusted due date — if an ultrasound moved your due date, this shows what conception date that implies.
- Filling in a missing data point — if you know when the baby was born but not when your last period was, this backfills a reasonable estimate.
How to Use This Calculator
- Choose a mode from the dropdown: From Last Period Date, From Due Date, or From Birth Date.
- For last period mode, enter the Last Period Start Date and Average Cycle Length (days) (defaults to 28, accepts 20–45).
- For the other two modes, enter the single relevant date.
- Select Calculate for the estimated conception date.
Related Calculations
Once you have an estimated conception date, run it forward through the Due Date Calculator. To estimate ovulation directly instead of conception, see the Ovulation Calculator.