How to use this calculator
- Enter or choose i know my.
- The most likely conception date updates as you type. Use Copy result or Share to keep it.
Method
Ovulation/conception = due date − 266 days (= cycle-adjusted LMP + 14 days); Fertile window = ovulation − 5 days to ovulation
- 266 days
- 38 weeks from conception to the due date
- fertile window
- the six days ending on ovulation (Wilcox et al., 1995)
Worked example
- Due date 15 January 2027 − 266 days = 24 April 2026 (estimated ovulation).
- Fertile window: 5 days before, 19 April, to 24 April 2026.
- Pregnancy is dated from 280 days before the due date: 10 April 2026.
A due date of 15 January 2027 points to conception around 24 April 2026, from intercourse most likely between 19 and 24 April.
What the result means
The date is an estimate with a margin of several days either way: ovulation timing varies from cycle to cycle, and sperm can survive for days before ovulation. A due date set by an early ultrasound gives a better estimate than one from period dates.
Assumptions and limits
- Ovulation and conception 266 days before the due date (14 days after a cycle-adjusted LMP).
- Fertile window of six days ending on the day of ovulation (Wilcox, Weinberg and Baird, 1995).
- Cannot establish paternity or the exact day of intercourse.
- Less reliable with irregular cycles or a due date based only on period dates.
Questions
Can I find the exact day I conceived?
Not from dates alone. Intercourse up to five days before ovulation can lead to pregnancy, so several days are equally plausible.
Is conception the same as implantation?
No. Implantation happens roughly a week after fertilisation; pregnancy tests turn positive after implantation.
Sources
Please note: This is a general screening estimate for adults, not medical advice or a diagnosis. Talk to a qualified health professional about your health.