SamCalculator

Pregnancy Conception Calculator

Estimate likely conception dates based on due date, last menstrual period, or ultrasound date.

Evidence-Based EstimationMobile FriendlyInstant ResultsEducational Use Only

Last reviewed May 2026 · Conception timing is an estimate — exact day cannot be determined without lab work.

Calculate based on

The estimated date of delivery (EDD) from your provider.

What Is Conception?

Conception is the moment a sperm fertilises an egg, forming a single-cell zygote. It usually happens in one of the fallopian tubes within roughly 12–24 hours after ovulation — the brief window when the egg is viable.

From a clinical perspective, conception is rarely observed directly. Instead, healthcare providers estimate the conception date by working backwards from a known reference point: a due date from a dating scan, the first day of the last menstrual period (LMP), or a measured gestational age. The convention is due date − 266 days, which assumes a 28-day cycle with ovulation on day 14.

How Doctors Estimate Conception Dates

Three reference points are commonly used, and they map directly to the three modes in this calculator:

Due date

Conception ≈ due date − 266 days. Easiest when you already have a confirmed EDD.

Last menstrual period

Conception ≈ LMP + 14 days. Standard Naegele approximation; assumes a 28-day cycle.

Ultrasound

Conception is back-derived from the gestational age at the scan, then projected to LMP and due date.

ACOG considers a first-trimester ultrasound (between 6 and 14 weeks) the most accurate way to date a pregnancy. After 22 weeks, ultrasound dating becomes less reliable and the original LMP-based estimate is usually preserved.

Ovulation and Fertility, Explained

Ovulation is the release of a mature egg from the ovary, triggered by a surge in luteinising hormone (LH) about 14 days before the next expected period. In a 28-day cycle this lands on day 14, but cycles of 24–35 days are still normal — so the actual ovulation day can vary considerably.

The fertile window opens roughly five days before ovulation and closes the day of ovulation. That's because sperm can survive in fertile cervical mucus for up to ~5 days, while the egg itself only lasts 12–24 hours after release. This is why the calculator shows two intercourse windows: a high-probability 3-day window and an extended 5-day window.

How Accurate Are Conception Calculators?

For a natural pregnancy, no calculator can pinpoint a single day of conception. The published convention of due date − 266 days is a population average. Real pregnancies vary because:

  • Cycle length varies cycle-to-cycle by ±2–5 days in most women.
  • Ovulation can shift forward or backward within the same cycle.
  • Sperm can survive up to five days before fertilising the egg, so the day of intercourse is not the day of conception.
  • Implantation occurs 6–12 days after fertilisation — different again from "conception".

Only laboratory-confirmed events such as IUI, IVF transfer, or LH-monitored cycles give a single known fertilisation day. For everyone else, a several-day window is the most a calculator can honestly provide — and the windows here reflect that.

Due Date vs. Ultrasound Accuracy

If your LMP-based due date and your first-trimester ultrasound disagree by more than 5–7 days, ACOG recommends using the ultrasound. The first-trimester scan measures crown-rump length, which correlates tightly with gestational age. After 22 weeks, the natural growth variation between fetuses widens, so ultrasound dating becomes less precise.

For LMP-based estimates to be accurate you need: regular ~28-day cycles, a clearly remembered LMP date, and no recent hormonal contraception. Many women — including those with PCOS, irregular cycles, or postpartum breastfeeding — get more reliable conception estimates from an ultrasound than from LMP.

Frequently Asked Questions

The standard clinical convention is: conception = due date − 266 days. This assumes a 28-day menstrual cycle with ovulation on day 14. The calculator also derives ovulation from LMP (LMP + 14 days) or from an ultrasound's gestational age, depending on what you enter.

Yes, this is the most common reverse-calculation method. If your due date is well-established (especially by a first-trimester ultrasound), subtracting 266 days gives a reliable estimated conception date — but it remains an estimate, not an exact day.

Yes, and a first-trimester ultrasound (6–14 weeks) is generally the most accurate way to date a pregnancy. The sonographer measures crown-rump length, which maps tightly to gestational age. From that gestational age, conception is back-derived as roughly 2 weeks (14 days) before the implied LMP.

Only in assisted-reproduction cycles where the timing is laboratory-controlled — IUI, IVF transfer, or LH-monitored ovulation cycles. For natural conception, the actual day cannot be determined retrospectively. The calculator's windows reflect that real uncertainty.

Sperm can survive in the female reproductive tract for up to about five days in fertile cervical mucus, with three days being typical. This is why the fertile window opens five days before ovulation, not just on ovulation day.

The most probable window is conception date ± 2 days — a tight 5-day band around the standard estimate. The possible window is ± 5 days (an 11-day band), reflecting the natural variability in cycle length and ovulation timing across real cycles.

Yes. Naegele's Rule (the LMP + 14 convention) assumes a 28-day cycle. If your typical cycle is 26 or 32 days, ovulation actually happens earlier or later — and so does conception. For irregular cycles, a first-trimester ultrasound gives a much more reliable estimate than LMP.

No. Conception (fertilisation) happens in the fallopian tube within 12–24 hours of ovulation. The resulting zygote then travels to the uterus and implants in the uterine lining about 6–12 days later. Some early-pregnancy bleeding is implantation bleeding — distinct from conception.

For IVF transfers and IUI cycles, your fertility clinic gives you the exact day. Those don't need a statistical estimator. This calculator is built for naturally-conceived pregnancies where the exact day isn't known — or where you want to reverse-engineer a likely day from your due date.

No. This is an educational estimation tool. It does not replace personalised advice from your obstetrician, midwife, or family-medicine provider, who can interpret your specific situation including cycle history, scan measurements, and any fertility considerations.

References

  1. 1. American College of Obstetricians and Gynecologists (ACOG). Committee Opinion No. 700: Methods for Estimating the Due Date. Obstetrics & Gynecology, 2017. acog.org
  2. 2. Centers for Disease Control and Prevention (CDC). Fertility and Pregnancy. cdc.gov/reproductive-health
  3. 3. Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD). Preconception Care & Prenatal Care. nichd.nih.gov

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