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Pregnancy Conception Calculator

Work backwards from a due date, last period or ultrasound to estimate the likely conception date and the fertile window that led to it.

Formulas last reviewed: September 2026

Disclaimer: Results are general estimates for information and education only. They are not medical advice, a diagnosis or a treatment plan. Please talk to a doctor or qualified health professional before making health, diet, exercise or pregnancy decisions.

Can you work out when you conceived?

Most people cannot tell the exact day they conceived. But you can estimate it by working back from the due date, your last period or an ultrasound dating scan.

Conception is usually about two weeks after the first day of the last period, when the egg is released and fertilised within about 12–24 hours.

How it is calculated

Most likely conception = due date − 266 days
Conception window = 11 to 21 days after the first day of the last period

Sperm can survive up to 5 days, so intercourse in the days before ovulation can also lead to pregnancy. The calculator therefore shows a wider window of possible intercourse dates.

Worked example

Due date 8 October 2026

Last period (calculated)
Thu, 1 Jan 2026
Most likely conception
Thu, 15 Jan 2026
Conception window
12 Jan – 22 Jan 2026
Intercourse window
7 Jan – 22 Jan 2026

Why the date is a range

  • Ovulation does not always happen on day 14 — it varies with cycle length.
  • Fertilisation can follow intercourse by several days.
  • Ultrasound dating in the first trimester is accurate to within about 5–7 days.
  • Babies of the same conception date can still be born weeks apart.

Frequently asked questions

Is the conception date the same as the fertilisation date?

In everyday use, yes. Fertilisation happens within about a day of ovulation. Implantation follows about 6–10 days later.

Why does my scan date differ from my conception date?

Scans measure the baby's size and estimate gestational age from the last period. The conception date is a separate estimate two weeks earlier.

Can this be used to find paternity?

No. The dates are estimates with a window of about a week. Only a DNA test can establish paternity.

What if I had IVF?

With IVF the conception date is known: for a day-5 embryo, the due date is the transfer date + 261 days. Use the due date calculator's IVF option.