Conception Date Estimate

Conception usually occurs around ovulation — roughly 14 days after the start of the last menstrual period in a 28-day cycle. From LMP, this calculator adds 14 days. From a known due date, it subtracts 266 days (280-day gestation minus the 14-day LMP offset).

Actual fertilization timing varies with cycle length, sperm survival, and IVF protocols. Ultrasound dating in early pregnancy is far more reliable than backward math from LMP alone.

Example: LMP January 10, 2026 → estimated conception around January 24, 2026. EDD October 17, 2026 → same conception estimate via the 266-day back-calculation.

Conception is a window, not a moment, and the width of that window is the honest answer. Sperm can remain viable in the reproductive tract for up to about five days while an egg is fertilisable for roughly 12 to 24 hours after ovulation, so intercourse days apart can produce the same conception date — which is why this estimate is best read as a range of several days.

The day-14 assumption is the weakest part of the calculation. Ovulation is more reliably counted backwards from the next period than forwards from the last one, because the luteal phase is relatively stable at about 12 to 14 days while the follicular phase varies. In a 32-day cycle, ovulation is nearer day 18 than day 14.

For a more precise estimate, use ovulation evidence rather than calendar math. A positive luteinising hormone test typically precedes ovulation by about 24 to 36 hours, and a sustained basal body temperature rise follows it — both narrow the window far more than a cycle-length assumption can.

Legal and administrative questions about conception timing need a clinician, not a calculator. Estimated conception dates derived from due dates carry uncertainty of a week or more, which is rarely adequate for anything consequential; early ultrasound dating and, where relevant, formal testing are the appropriate tools.

Note that implantation happens later than fertilisation, usually six to ten days afterwards, and that is when pregnancy tests can begin to detect hCG. A negative test shortly after the estimated conception date says very little — this tool provides calendar math for context, not medical advice.

For IVF, use the IVF due date calculator instead — embryo age at transfer matters.

Implantation occurs several days after fertilization — the estimated conception window is biology’s best guess from due-date math, not a pinpoint ovulation timestamp from a home test.

Cycle length variability widens the conception window — irregular periods make LMP-based reverse dating less precise than early ultrasound.