Conception Date Calculator

Have you ever wondered exactly when your little one was conceived? Working backwards from your due date, this calculator estimates the conception date, your fertile window, and when your last menstrual period (LMP) likely occurred. It is a lovely way to piece together the timeline of a new life beginning — and a practical reminder of which week you were in when the pregnancy started.

This conception calculator is for expectant parents who already have a due date (from a period-based estimate or, better still, an ultrasound) and want to trace it back to the likely date of conception. People use it out of curiosity, to fill in a baby journal or memory book, or simply to understand the biology of their own cycle. It is a screening and information tool, not a way to establish anything medical or legal.

How conception is calculated

A full-term pregnancy lasts approximately 280 days (40 weeks) counted from the first day of the LMP. Because ovulation usually happens around day 14 of a textbook 28-day cycle, conception typically occurs about 266 days (38 weeks) before the due date — roughly two weeks after the LMP. This calculator therefore uses Conception Date = Due Date − 266 days, and works the estimated LMP out as LMP = Due Date − 280 days.

The fertile window

Sperm can survive in fertile cervical mucus for up to 5 days, while a released egg is viable for only about 12–24 hours after ovulation. Together these create a roughly 6-day fertile window — the 5 days before ovulation plus ovulation day itself — during which conception is possible. The day before ovulation and ovulation day carry the highest chance. This is why the conception date is best thought of as the centre of a short window rather than a single, certain day.

A worked example

Imagine your due date is 22 October 2026. Subtracting 266 days gives an estimated conception date of about 29 January 2026. Subtracting 280 days from the due date puts your estimated LMP near 15 January 2026. The fertile window around that conception date would span roughly 24 January to 30 January 2026 — the days when intercourse was most likely to have led to this pregnancy.

How conception week relates to your due date

Counting forward from conception, a full-term pregnancy runs about 38 weeks (266 days), which is why the due date sits roughly 38 weeks after the day sperm met egg. In gestational terms — the way doctors count, starting from your LMP — that same moment of conception falls in week 2 to week 3 of pregnancy. So if this calculator places your conception in late January, you were already being counted as around 2 weeks pregnant at that point, and your first missed period would have arrived a couple of weeks later. Understanding this offset explains why the "weeks pregnant" number always runs ahead of the time since conception.

Conception vs implantation vs fertilisation

It helps to keep three events separate. Fertilisation (conception) is the moment sperm meets egg — for everyday purposes the words are used interchangeably. Implantation happens 6–12 days later, when the fertilised egg embeds in the wall of the uterus; this is when pregnancy hormone (hCG) begins to rise, which is what a home pregnancy test detects. So a positive test confirms implantation has occurred, a little after the conception date this tool estimates.

Key tips and common notes

• Prefer an ultrasound-based due date. If your gynaecologist set your EDD from a first-trimester scan, use that date here — it is usually more accurate than a period-based due date, and so is the conception estimate that follows from it.
• Cycle length matters. If your cycles are longer or shorter than 28 days, you may have ovulated earlier or later than day 14, which shifts the true conception date by a few days.
• It is an estimate, not proof. Because the fertile window spans several days and ovulation timing varies, a calculated conception date cannot establish paternity — only DNA testing can do that.
• Use it gently. This is a fun, informational timeline. For anything that affects your pregnancy care, rely on your scans and your doctor.

This calculator provides an estimate for general interest and information only — it is not medical advice. For an accurate conception date and gestational age, please consult your doctor or gynaecologist, who can confirm dates with an ultrasound.