Pregnancy Conception Date Calculator – Estimate Date

Conception is usually estimated as 266 days (38 weeks) before the due date, or about 14 days after the first day of the last menstrual period (LMP). This calculator uses both methods to return a likely conception window.

When to use this calculator

You know your estimated due date and want a likely conception date.
You only know the first day of your last menstrual period.
You want to narrow down a conception window for personal records.

How it works

Pregnancy is conventionally counted as 280 days from LMP (40 weeks), with ovulation and conception around day 14 of an average 28-day cycle. So conception ≈ LMP + 14 days, or due date − 266 days. The calculator returns both the estimated date and a ±5-day window to reflect normal cycle variation.

Real-world examples

Due date 12 December → conception window centred on about 21 March.
LMP 1 January in a regular 28-day cycle → conception around 15 January.
Personal record use: linking a baby's birthday back to a likely conception window for family history.

Limitations

Assumes a regular 28-day cycle and ovulation on day 14 — many cycles vary.
Does not detect actual ovulation (only a tracker, ultrasound, or LH test does that).
Late or early ovulation can shift the real conception date by a week or more.
Always confirm pregnancy timing with a healthcare professional and ultrasound dating.

Frequently asked questions

How is the conception date calculated?

Either as 14 days after the first day of the last period, or as 266 days before the estimated due date. Both methods give similar results in a regular cycle.

How accurate is this estimate?

Within a few days for a regular 28-day cycle. For irregular cycles or unknown LMP, the estimate can drift by a week or more.

Does this predict ovulation?

Only as an average. For real ovulation timing, use ovulation predictor kits, basal body temperature charting, or a fertility-tracking app.

What if my cycle is longer or shorter than 28 days?

Conception happens about 14 days before your next expected period, not always on day 14. In a 32-day cycle, ovulation is closer to day 18; in a 24-day cycle, closer to day 10.

Should I use this for medical decisions?

No. Treat it as a personal estimate. Pregnancy dating, due dates, and any clinical decisions should come from a healthcare provider, not a web calculator.

Pregnancy Conception Calculator