Pregnancy Conception Calculator

What is a conception date?

A conception calculator estimates the day pregnancy most likely began — when the egg was fertilised. Parents ask out of curiosity, but the date also matters medically: it anchors the timeline doctors use to interpret early scans and screening results. Because the exact moment cannot be observed, every method gives an estimate, usually accurate within a few days for regular cycles.

How it works

This calculator offers two paths to the same answer:

• From your last period (LMP): conception ≈ LMP + 14 days. In a textbook 28-day cycle, ovulation — and therefore fertilisation — happens about two weeks after the period starts.
• From your due date: conception ≈ due date − 266 days. A full pregnancy lasts 280 days from the LMP but only 266 days from conception (40 weeks minus the 2 pre-ovulation weeks).

Both routes should land on roughly the same day; if they differ by more than a few days, the cycle was probably not a standard 28 days.

How to use this calculator

1. Choose "Last period" or "Due date" depending on which date you know.
2. Enter that date.
3. Read the estimated conception date — and remember it is the centre of a small window, not a single certain day.

Worked example

Last period began 1 January:

• Conception ≈ 1 Jan + 14 days = 15 January

Cross-check from the due date: 1 Jan + 280 days = 8 October; 8 Oct − 266 days = 15 January. Both methods agree.

Limitations and tips

• Ovulation varies: stress, illness and natural variation move ovulation earlier or later, so the ±14-day assumption is approximate. The fertile window actually spans about 6 days.
• Irregular cycles need scans: with irregular periods, early ultrasound dating beats any calendar maths.
• IVF is exact: for assisted reproduction the embryo transfer date is known precisely — no estimation needed.
• Do not over-interpret: a conception estimate cannot prove paternity timing or anything beyond a rough window; treat it as the helpful approximation it is.

Understanding the fertile window

Conception is only possible during the fertile window — roughly the 5 days before ovulation plus ovulation day itself — because sperm survive up to 5 days while the egg survives about 24 hours. That is why the estimate centres on ovulation day but the real window is wider. Couples trying to conceive are usually advised to have intercourse every 1–2 days through this window rather than aiming at a single day.