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Conception Date Calculator

Enter a due date or current gestational age to get LMP, conception window and key milestones.

Working backwards from a due date to when the child was conceived is one of the most common questions in an antenatal clinic. Whether the starting point is a due date or the current gestational age, the calculation runs back to the last menstrual period and estimates the conception day and window, laying out four key dates — useful both for lining up a timeline and for recalibrating after a dating scan.

On the conventions: the conception day is estimated from ovulation, taken as the last menstrual period plus the cycle length minus 14 days on a 28-day cycle, and actual ovulation floats by about two days either way; the fertile window runs from five days before ovulation to one day after, since sperm survive up to five days. Every result here is an estimate, and gestational age is confirmed by an early scan.

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How to use

  1. Enter either the due date or the current gestational age.
  2. Read the estimated last period and conception window.
  3. Check the four milestone dates.
  4. Treat the early scan as the authority on gestational age.

How it works

The difference between 280 and 266 days

Obstetrics has two starting points: due date = first day of LMP + 280 days (Naegele's rule, assuming ovulation about 14 days after the LMP); conception occurs on the ovulation day, so from conception to the due date is about 266 days. This tool does the inverse of both: LMP = due date - 280 days, conception date is about due date - 266 days. For a due date of March 24, 2027, the LMP back-solves to June 17, 2026 and conception to about July 1, 2026.

Why gestational weeks start from the LMP

Gestational weeks start from the first day of the LMP, not the conception date: at "4 weeks pregnant" the actual conception is only about 2 weeks ago. This is the unified convention of obstetrics and ultrasound reports, so back-solving by gestational weeks must account for it — this tool converts "current gestational weeks + leftover days" directly into days remaining to get the due date.

How large the estimation error is

The error must be stated clearly: sperm can survive several days in the body and ovulation timing fluctuates, so the back-solved conception date is a statistical estimate with errors of several days or more, giving a "conception window" rather than a single day. Any use needing precise conception timing (like legal or paternity determinations) must rely on medical and forensic means.

Calculation basis: the inverse of Naegele's rule (due date = first day of the last period + 280 days) — last period = due date − 280 days and conception ≈ due date − 266 days, with a fertile window from 5 days before ovulation to the day itself. The result is a statistical estimate and cannot establish the actual date of conception.

Code example

JavaScript Conception and due date from the last period 

const DAY = 86400000;

function conception(lmp) {
  const t = new Date(lmp).getTime();
  return {
    ovulation: new Date(t + 14 * DAY),      // ovulation day (28-day cycle)
    window: [new Date(t + 9 * DAY), new Date(t + 15 * DAY)],
    edd: new Date(t + 280 * DAY)            // due date (Naegele's rule)
  };
}

// Gestational age counts from the last period: 4 weeks pregnant ≈ 2 weeks after conception

Python The same formulas in Python 

from datetime import date, timedelta

def conception(lmp):
    t = date.fromisoformat(lmp)
    return {"ovulation": t + timedelta(days=14),
            "window": (t + timedelta(days=9), t + timedelta(days=15)),
            "edd": t + timedelta(days=280)}

# Gestational age counts from the last period; an early ultrasound is the definitive dating

FAQ

Can the conception date be computed precisely?

No. Here it's estimated as 266 days before the due date; actual conception may differ by several days (sperm survive 3-5 days, ovulation timing fluctuates), so the tool gives a "conception window," not a day.

Why 266 days and not 280?

280 days starts from the first day of the LMP (assuming ovulation about 14 days later); conception happens on the ovulation day, so from conception to the due date is about 280 - 14 = 266 days. Different starting points, no contradiction.

How exactly do I count gestational weeks?

From the first day of the LMP, not the conception date. So at "4 weeks pregnant" the actual conception is about 2 weeks ago. This is the unified convention of obstetrics and ultrasound reports — don't convert by the conception date when reading reports.

What if an early ultrasound changes the due date?

Use the corrected due date and re-enter it. An early-pregnancy (before 12 weeks) ultrasound by CRL is most accurate, and doctors usually adjust the due date accordingly.

Can this result determine the conception time?

No. The estimation error can be several days or more; any use needing precise conception timing must rely on medical examination and forensic means, not on back-solving the due date.

How is it computed for IVF or a C-section?

IVF is computed from the transfer date (e.g. a day-3 embryo is roughly transfer date + 17 days in the LMP convention); follow the reproductive center's due date. A C-section date is decided by the doctor based on mother and baby, unrelated to this tool.

Is the due date or gestational week I enter uploaded?

No. The date is only added and subtracted locally in the browser, not sent or saved; the site does no user identification and has no endpoint aggregating this data. History stays in your browser and is clearable in one click.