Conception Date Calculator
Estimates how many days ago conception occurred from a reference point — your due date, last menstrual period, or an ultrasound dating scan — entered as days since that reference (negative if it is a future due date).
Last updated: May 2026
About this calculator
Conception — fertilisation of the egg — typically occurs around ovulation, approximately 14 days after the start of a 28-day menstrual cycle. The calculator supports three methods. From a due date it subtracts 266 days (38 weeks of actual fetal development): conceptionDate = dueDate − 266 days. From the last menstrual period (LMP) it adds 14 days plus an ovulation offset for non-standard cycle lengths: conceptionDate = LMP + 14 days + (cycleLength − 28) days. From an ultrasound gestational age it reverses the due-date logic: conceptionDate = ultrasoundDate − (ultrasoundWeeks × 7) + 266 days. All three methods account for the biological gap between the LMP-based gestational clock and actual fertilisation, which is consistently about two weeks.
How to use
Suppose you use the LMP basis and your last period started 60 days ago with a 30-day cycle. Conception is about 14 days after the LMP, adjusted for the 2-day-longer cycle: 60 − 14 − (30 − 28) = 44 days ago. Result: "Estimated conception about 44 days ago". With the Due Date basis, enter a negative number (e.g. −100 for a due date 100 days away); conception ≈ referenceDaysAgo + 266.
Frequently asked questions
How do I calculate my conception date from my due date?
Subtract 266 days (38 weeks) from your due date to estimate conception. This figure represents the average duration from fertilisation to birth, as opposed to the 280-day (40-week) gestational clock that counts from the LMP. For example, a due date of 1 April 2025 points to a conception date around 9 June 2024. Keep in mind this is a statistical average — actual conception windows span a few days around ovulation.
Why does cycle length affect the calculated conception date?
Standard obstetric dating assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, ovulation shifts accordingly, moving the likely conception window. For a 35-day cycle, ovulation occurs around day 21, so the calculator adds an extra 7 days to the baseline 14-day offset. Accurate cycle length input therefore meaningfully improves the precision of your estimated conception date.
Can an ultrasound give a more accurate conception date than LMP?
Yes, a first-trimester ultrasound (ideally between 8 and 13 weeks) is considered the most reliable dating tool because it measures actual embryo or fetal size rather than relying on a recalled LMP date. The calculator uses the gestational age assigned by the ultrasound, subtracts those weeks from today, and adds 266 days to arrive at an estimated conception date. Ultrasound dating is especially useful when cycles are irregular or the LMP is unknown.