Conception Date Calculator

How Conception Date Is Estimated From a Due Date

This calculator runs the standard due-date formula in reverse. A full-term pregnancy is conventionally estimated at 266 days measured from conception, so subtracting 266 days from a known or expected due date gives an estimated conception date. This is the mirror image of Naegele's Rule, which adds 280 days to the first day of the last menstrual period (LMP) to estimate a due date going forward.

Why 266 Days Here, But 280 Days on the Due Date Calculator

Both numbers describe the same roughly 40-week pregnancy — they just start counting from two different reference points. LMP-based dating begins on the first day of the last period, which in a typical 28-day cycle falls about 14 days before ovulation. Conception-based dating begins at ovulation and fertilization itself, skipping that 14-day head start entirely. So 280 minus 14 equals 266: the same pregnancy length, counted from a starting point two weeks later. This calculator is not using an unrelated formula — it is running the pregnancy-calculator's own relationship in reverse, from the due-date end.

Conception vs. Ovulation: Why They're Shown as the Same Date

Ovulation is when a mature egg is released from the ovary; conception is the moment that egg is fertilized by sperm. In practice, these happen very close together — a released egg is viable for roughly 12 to 24 hours, and fertilization typically occurs within that window. Because the gap is small relative to a pregnancy measured in weeks, standard due-date math treats the two as a single event, which is why this calculator shows one estimated date for both.

Worked Example

If a due date is December 1, subtracting 266 days lands on March 9 of the same year as the estimated conception (and ovulation) date. Subtracting 280 days instead — the LMP-based version — lands on February 23, roughly 14 days earlier, which is the estimated first day of the last menstrual period for that same pregnancy.

Why This Is an Estimate, Not a Precise Date

Actual pregnancy length varies from person to person, and this calculator assumes an average-length, 266-day pregnancy counted from conception. Sperm can also survive inside the reproductive tract for several days before an egg is released, so fertilization doesn't always happen on the exact day of ovulation. Treat the result as a useful planning estimate, not a certified date — an early ultrasound is generally considered more precise for dating a pregnancy.

A Brief History of Due-Date and Conception-Date Estimation

The 280-day, last-menstrual-period-based due-date rule this calculator works backward from is commonly called Naegele's Rule, after the German obstetrician Franz Karl Naegele, who published it in an obstetrics textbook in the early 19th century. Historians of medicine have since pointed out that the rule likely didn't originate with Naegele himself — earlier versions appear in the work of the Dutch physician Herman Boerhaave roughly a century earlier, and Naegele's own writing credited that earlier source rather than claiming the idea as original. The name stuck regardless, and the rule is still the standard first-pass estimate used in obstetrics today, more than two centuries later.

The rule's simplicity is also its known limitation: it assumes a textbook 28-day cycle with ovulation on day 14, which doesn't hold for everyone, and it was formalized long before ultrasound existed. Modern obstetric practice still starts from a Naegele's-Rule-style estimate but generally treats an early ultrasound measurement as more reliable once one is available, precisely because it measures the fetus directly rather than relying on a population-average cycle length — a reminder that this calculator, like the rule it's built on, produces a statistical estimate rather than a precise, individually measured date.

Common Conception-Date Mistakes

Entering the first day of the last menstrual period instead of an actual or expected due date is the most common input mistake — this calculator expects a due date and subtracts 266 days from it, so feeding it an LMP date instead produces a conception estimate that's roughly two weeks off. Confusing this tool's output with the separate due-date calculator's own inputs is another frequent mix-up, since the two calculators run the same underlying relationship in opposite directions and aren't interchangeable. Using an estimated or roughly-remembered due date, rather than one confirmed by a clinician or an early ultrasound, also compounds whatever uncertainty already exists in the 266-day estimate itself.

Conception Date Terms You Should Know

Gestational Age — pregnancy length measured from the first day of the last menstrual period, the standard convention used by most due-date calculators.

Fetal Age — pregnancy length measured from conception, roughly two weeks shorter than gestational age at any given point.

Fertilization Window — the several-day span around ovulation during which conception can occur, since sperm can survive for days while the egg is viable for only about a day.

Naegele's Rule — the standard formula for estimating a due date by adding 280 days to the first day of the last menstrual period.