Reverse Due Date Calculator — Find Conception Date from Due Date

Why Due Dates Are Always Estimates

A due date is a population-level reference point, not a scheduled arrival. Only about 5% of birthing people deliver spontaneously on their calculated due date, and roughly 80% deliver within a ±2-week window around it. Variation comes from normal biological differences in implantation timing, menstrual-cycle length, fetal growth patterns, and individual factors we do not fully measure with simple calculators. Your prenatal care team will adjust your official dating based on first-trimester ultrasound crown-rump measurement when needed, which is the most accurate reference window we have.

Method 1 — Naegele Rule (LMP + 280 Days)

Method 2 — Conception Date

If you know the approximate date of ovulation, intercourse, or IUI or IVF fertilization, a conception-date estimate adds 266 days (38 weeks, the average duration of human gestation from fertilization to term) instead of 280. This removes the two-week LMP-to-conception offset, so the raw calculation does not depend on cycle length. Keep in mind that sperm can survive inside the female reproductive tract for several days, so if you are estimating from intercourse the actual fertilization window can span multiple days.

Method 3 — IVF Transfer

Cycle Length and LMP Dating

Naegele rule assumes a 28-day cycle. For people with consistently longer or shorter cycles, a simple linear adjustment (shifting the estimate by [cycle length − 28] days) can be applied. However, because luteal-phase length is relatively stable across cycle lengths (about 14 days) while follicular phase varies, adjusting LMP-based estimates purely for cycle length is an approximation at best. For irregular cycles, conception-date or first-trimester-ultrasound dating is generally more reliable.

Trimester Split

Three standard trimesters are used for reference and prenatal-care scheduling. The first trimester runs from LMP through week 12 + 6 days and covers the period of highest sensitivity for embryological development. The second trimester spans week 13 through week 27 + 6 days and is the typical window for anatomical screening, prenatal sugar screening, and fetal-growth surveys. The third trimester runs from week 28 through delivery, with the viability reference point commonly placed at 24 weeks and the term reference window at 37–42 weeks.

What a Reverse Due Date Calculator Does

A reverse due date calculator works backwards from a known due date to estimate the conception date and the LMP equivalent. The standard Naegele rule runs forward (LMP plus 280 days equals due date); the reverse calculation inverts the arithmetic. Subtract 266 days from the due date to estimate the conception date, or subtract 280 days to estimate the LMP equivalent. The two differ by 14 days because the Naegele convention places conception around day 14 of a 28-day cycle. This reverse calculation is useful when the due date is known (for example, set by a first-trimester ultrasound) but the LMP is uncertain, or when planning a pregnancy around a target delivery window.

When to Use the Reverse Calculation

The reverse calculation has two common use cases. First, when the due date has been set by a first-trimester ultrasound but the LMP is uncertain (irregular cycles, recent hormonal contraception, or simply no tracked LMP), the reverse calculation gives an estimated LMP equivalent and conception date for personal records. Second, when planning a pregnancy around a target delivery window — for example, aiming to deliver in a particular month — the reverse calculation shows the conception window that would land near that target. In both cases, the reverse calculation produces estimates, not exact dates, because individual gestation length varies by roughly plus or minus 2 weeks around the 280-day reference.

Reverse Calculation From a Scheduled Delivery Date

A common forward-planning use case is a scheduled cesarean delivery or planned induction. If the delivery is scheduled for a specific date, the reverse calculation shows the conception window that would have produced that delivery date under the standard Naegele assumption. In practice, of course, you cannot plan conception around a future scheduled delivery — the reverse calculation is more useful for understanding the timeline of an existing pregnancy than for planning one. For a planned delivery in an existing pregnancy, the reverse calculation tells you the gestational age that will be reached on the delivery date, which helps confirm that the planned date falls within the term window (37 to 42 weeks).

Reverse Calculation vs Forward Calculation

The forward calculation (LMP plus 280 days, or conception plus 266 days) is the standard pregnancy dating method because LMP and conception are typically the known inputs. The reverse calculation (due date minus 266 or 280 days) inverts the same arithmetic. Both methods use the same population averages and have the same margin of roughly plus or minus 2 weeks for individual gestation length. The reverse calculation does not produce a more precise estimate — it just produces an estimate of a different date. If the due date was set by a first-trimester ultrasound, the reverse-calculated LMP equivalent will be consistent with the ultrasound-derived gestational age, which is the most accurate reference in early pregnancy.

Interpreting the Reverse-Calculated Conception Date

The reverse-calculated conception date is an estimate of the fertilization date, not a guarantee. Because sperm can survive 3 to 5 days in the reproductive tract, intercourse that led to conception may have happened up to 5 days before the reverse-calculated conception date. Equally, ovulation may have happened a day or two off the assumed day-14 timing, shifting the actual conception date. If you are trying to identify the paternity window or understand which intercourse event led to conception, view the reverse-calculated conception date as the center of a 5-to-7-day window rather than a precise day. A first-trimester ultrasound provides the most accurate reference for confirming the gestational age that underlies the reverse calculation.

Practical Reference Tips

• Subtract 266 days from a known due date to estimate the conception date. Subtract 280 days to estimate the LMP equivalent.
• Use the reverse calculation when the due date is known (set by ultrasound) but LMP is uncertain, or when planning around a target delivery window.
• View the reverse-calculated conception date as the center of a 5-to-7-day window, because sperm can survive 3 to 5 days and ovulation timing varies.
• If the due date was set by a first-trimester ultrasound, the reverse-calculated LMP equivalent will be consistent with that ultrasound-derived gestational age.
• For a planned cesarean or induction, the reverse calculation shows the gestational age that will be reached on the planned date — confirm it falls within the 37 to 42 week term window.