Due Date by Conception Date — Calculate from Conception

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.

Why Conception Date Dating Skips the LMP Guesswork

The standard Naegele rule dates a pregnancy from the first day of the last menstrual period (LMP) by adding 280 days. That figure bakes in a 14-day assumption between LMP and ovulation, which introduces uncertainty whenever a cycle does not match the 28-day pattern. When the conception date is known, you can skip that 14-day guess entirely. The fertilization-to-term gestation averages 266 days (38 weeks), so the due date equals conception date plus 266 days. This method is more precise than LMP-based estimation whenever the conception date is documented to the day — most reliably with IVF, but also for individuals who tracked ovulation with predictor kits or body signals and identified a single fertilization event.

How to Determine Your Conception Date

The conception date is the date fertilization happened — the sperm met the egg. Because the egg survives 12 to 24 hours after ovulation and sperm survive 3 to 5 days in the reproductive tract, fertilization typically happens on the ovulation day or within a day of it. Three common ways to pin down the conception date:

• Ovulation predictor kits (OPKs): A positive OPK indicates the luteinizing hormone (LH) surge, which typically precedes ovulation by 24 to 36 hours. Conception most likely happens 1 to 2 days after the positive test.
• Basal body temperature (BBT) shift: A sustained BBT rise of 0.2 to 0.5 °C (0.4 to 0.9 °F) confirms ovulation has happened. The conception date is most likely the day of the shift or the day before.
• Cervical mucus peak day: The last day of clear, slippery, stretchy (raw-egg-white) mucus is typically the ovulation day or very close to it.
• IVF or IUI documentation: With IVF, the fertilization date is documented in the laboratory. With intrauterine insemination (IUI), the insemination date is documented and ovulation typically follows within 24 hours.

If you only know the date of intercourse, the conception date is less precise because sperm can wait up to 5 days before fertilization. In that case, view the intercourse date as the upper bound of the conception window.

Conception vs LMP — Why the Two Methods Converge

The conception-date method (conception + 266 days) and the Naegele method (LMP + 280 days) are mathematically consistent. The Naegele estimate adds 280 days to LMP, of which 14 days run from LMP to estimated ovulation. If ovulation happened on day 14 of a 28-day cycle, then conception also happened around day 14, and adding 266 more days reaches the same date as adding 280 from LMP. The two methods diverge only when the actual cycle does not match the 28-day / day-14-ovulation pattern — which is exactly when the conception-date method is more accurate. If you tracked ovulation and know the conception date, use the conception method; if you only know LMP, use Naegele with a cycle length adjustment.

IVF — When the Conception Date Is Known Exactly

IVF is the case where the conception date is documented to the day, because fertilization happens in the laboratory. For IVF, the due date is typically calculated from the embryo transfer date and the embryo age (Day 3 or Day 5) rather than from the fertilization date directly, but the two routes are equivalent. A Day 5 blastocyst transferred on March 1 was fertilized 5 days earlier, on February 24, so due date equals February 24 plus 266 days, which equals March 1 plus 261 days — the same result. The IVF transfer method is preferred in practice because the transfer date and embryo age are the most cleanly documented information in the IVF record.

When the Conception Date Is Only Approximate

If your conception date comes from intercourse timing or a single OPK positive, the date is approximate within a few days rather than exact. Sperm can survive 3 to 5 days in the reproductive tract, so intercourse 4 days before ovulation can still result in fertilization on the ovulation day. To reflect this uncertainty, view the calculated due date as the center of a window rather than a precise day, and rely on the first-trimester ultrasound (crown-rump length measurement) to refine the dating. The ultrasound is the most accurate reference in early pregnancy, with a margin of about 5 to 7 days at 8 to 12 weeks.

Practical Reference Tips

• If you tracked ovulation with OPKs or BBT, the conception date is most likely 1 to 2 days after the LH surge or on the day of the BBT shift.
• Use the conception-date method (conception + 266 days) when the conception date is known to within a day. Use Naegele with cycle length adjustment when only LMP is known.
• For IVF, use the transfer-date method (transfer + 266 − embryo age days) — it is equivalent to conception + 266 but uses the most cleanly documented dates.
• Confirm the dating at your first prenatal ultrasound. The crown-rump length measurement is the most accurate reference in early pregnancy.
• If the conception date is approximate (intercourse timing only), view the calculated due date as the center of a window rather than a precise arrival date.