When did I conceive?
Down to a 3-day window.

Field guide

How a conception calculator works.

“Conception” is the day a sperm fertilises an egg. It corresponds, for practical purposes, to the day of ovulation. Because that day can be calculated either forwards from your last menstrual period (LMP) or backwards from your estimated due date (EDD), this calculator supports both routes and produces the same timeline either way when the inputs agree.

From the LMP

In a textbook 28-day cycle, ovulation occurs around day 14. More generally:

The constant 14 is the luteal phase: the time between ovulation and the start of the next period. The luteal phase is more stable across people than the follicular phase (the time before ovulation), so the convention treats it as a fixed 14 days and varies the follicular phase with cycle length.

From the EDD

Pregnancy is 266 days from conception (38 weeks of embryonic development), or equivalently 280 days from LMP (40 weeks of gestational age). So:

The cycle-length adjustment to LMP is needed because the EDD already incorporates whatever cycle length is being assumed. If your obstetrician set your EDD using a 28-day cycle assumption, the back-calculated LMP will only match your real LMP if your cycle is actually 28 days.

How accurate is a conception calculator?

About ±2–3 days for a typical input. The math is exact, but the inputs aren't:

• LMP recall. Most people's memory of the exact first day of their period is off by 1–3 days. That noise propagates straight into the conception estimate.
• Cycle variability. Even a person with a “regular” cycle ovulates 3–5 days earlier or later than average from one cycle to the next. The calculator can't see that; it assumes the cycle length you enter is the cycle on which conception happened.
• Luteal-phase variability. The 14-day luteal phase is an average. Individuals vary from about 10 to 16 days. A shorter luteal phase pushes the calculated conception date later than the real one; longer pushes it earlier.
• Sperm survival. Sperm can fertilise an egg up to 5 days after intercourse, so even a perfectly timed conception calculation can't pinpoint the moment of intercourse, only the moment of fertilisation.

All of which is why the calculator surfaces a 3-day window, not a single point. The highlighted ±1-day band is the realistic resolution this kind of calculation can achieve.

The fertile window

Adjacent to conception is the fertile window, the days during which intercourse can lead to pregnancy. Because sperm survive in the reproductive tract:

That's a 6-day band ending on the day of ovulation. For people trying to conceive, the highest-probability days are the two days before ovulation, when sperm are already in place when the egg is released.

Conception vs. implantation

People often conflate these. Conception is fertilisation: sperm meets egg, usually in the fallopian tube. Implantation is when the resulting blastocyst attaches to the uterine lining, which happens 6–12 days later (peak at day 8–9).

Implantation is the event that triggers measurable hCG production, which is what home pregnancy tests detect. So a positive test typically means you've been pregnant (in the conception sense) for about 10–14 days already.

From conception to due date

Once you have the conception date, the rest of the timeline follows mechanically:

• Day 0: conception (fertilisation in the fallopian tube).
• Day 6–12: implantation in the uterine lining.
• Week 4 (gestational): first missed period, positive home pregnancy test.
• Week 6: fetal heartbeat detectable on transvaginal ultrasound.
• Week 12: first-trimester screening (NT scan).
• Week 20: anatomy scan.
• Week 40: estimated due date (266 days after conception, 280 days after LMP).

Worked example

Last period started 1 January 2026, cycle length 28 days. Then:

• Conception ≈ 15 January 2026 (LMP + 14)
• 3-day window: 14–16 January 2026
• EDD = 8 October 2026 (LMP + 280)

Switch to a 32-day cycle and the conception date becomes 19 January 2026: four days later; and the EDD becomes 12 October 2026.

Disclaimer

This calculator is for educational use only and is not a substitute for professional medical advice. Conception timing and pregnancy progression are highly individual. Discuss specific dates with your obstetrician, midwife, or family physician, particularly when paternity or precise gestational age has clinical consequences.