Conception Date Calculator
Find the conception date behind a due date.
Conception Date Calculator
Work backwards from a due date or last period to the most likely conception date and intercourse window.
Wed, 5 August 2026
2026-08-05
- Likely intercourse window
- 2026-07-31 → 2026-08-06
- Estimated last period
- 2026-07-22
- Estimated due date
- 2027-04-28
Conception is dated to fertilisation, about two weeks after the first day of the last period. The window covers the days intercourse could have led to it.
This is an estimate based on calendar dates, not a medical diagnosis or a dating scan. Only about 4% of babies arrive on their estimated due date. Speak to your midwife, GP, or obstetrician about dating and any symptoms that worry you.
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What This Calculator Does
The Conception Date Calculator estimates the date of fertilisation from either an estimated due date or the first day of your last period. It also shows the intercourse window that could have led to conception, plus the implied last period and due date.
How It Works
A pregnancy is dated from the last menstrual period, about two weeks before conception. Working backwards, conception is roughly 266 days before the due date, or 14 days after the LMP in a 28-day cycle. Because sperm survive up to five days, the calculator also reports a window from five days before that date to one day after.
Example
For a due date of 8 October 2026, the estimated conception date is 15 January 2026, the likely intercourse window is 10–16 January, and the implied last period is 1 January 2026. With a 30-day cycle starting 1 May 2026, conception is estimated at 17 May 2026.
Formula
Conception = Due date − 266 daysConception = LMP + (cycle length − 14)Assumes a 14-day luteal phase; adjust the cycle length rather than the luteal phase.
Window = conception − 5 days to conception + 1 dayReading the Results
Conception date
The estimated day of fertilisation. It is a statistical midpoint: ovulation, and therefore conception, can occur several days away from it.
Likely intercourse window
The days on which intercourse could plausibly have led to this pregnancy, allowing for sperm survival and egg viability.
Estimated last period
The LMP implied by the dates, useful when you are asked for it at a first antenatal appointment.
Sources and References
ACOG Committee Opinion 700 — Methods for Estimating the Due Date
266 days from conception to due date
The 280-day LMP rule and the 266-day conception rule describe the same 40-week pregnancy.
NICE NG201 — Antenatal care
Dating scan at 11–14 weeks
Scan dating takes precedence when it differs from calendar estimates.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
How is the conception date calculated?
Subtract 266 days from the estimated due date, or add 14 days to the first day of the last period in a 28-day cycle. The result is an estimate of fertilisation, not a precise timestamp.
How precise is a conception date?
It is typically accurate to within about a week, and less so with irregular cycles. Ultrasound dating, which measures the embryo directly, is more precise than any calendar calculation.
Is the conception date the same as the ovulation date?
Very close. Fertilisation happens within roughly 24 hours of ovulation, so the two dates are usually the same or one day apart.
Can this calculator establish paternity?
No. It only estimates dates. Paternity is established by DNA testing, not by calendar arithmetic.