Home › Tools › Conception date calculator

Conception date calculator

Find out roughly when your baby was conceived. Enter your due date or the first day of your last period, and we will show your most likely conception date, the days sex could have led to pregnancy and when implantation usually happens.

Important: This is an estimate. Ovulation can vary from cycle to cycle and sperm can live for up to five days, so the exact day of conception is rarely known. This tool cannot be used to establish who the father is. Your care team's dates take priority.

Your dates are processed in this page and are not sent to our server or saved by this tool.

How the conception date is worked out

A pregnancy lasts about 280 days, or 40 weeks, from the first day of your last period. Conception happens about two weeks into that count, when an egg is released and fertilised. Counting back from the due date, conception is about 266 days (38 weeks) earlier. If you start from your last period instead, we add your cycle length minus 14 days, because ovulation usually happens about 14 days before your next period is due.

Conception, ovulation and implantation

Ovulation: an egg is released from the ovary. It survives for around 12 to 24 hours.
Conception (fertilisation): a sperm meets the egg, usually within a day of ovulation.
Implantation: the fertilised egg attaches to the lining of the womb, usually 6 to 12 days after ovulation. Some people notice light spotting or cramps around this time, but many notice nothing.
A positive test: a pregnancy test detects the hormone hCG, which begins to be made after implantation. The earliest sensitive tests can show a result about 10 days after ovulation, and most are reliable from the day your period is due.

Why the date is not exact

Sperm can survive in the body for up to five days, so sex before ovulation can lead to pregnancy. That is why the calculator shows a window of days as well as a single date. Ovulation can also come earlier or later than expected, especially with irregular cycles, stress or illness. A dating scan, usually offered in the first trimester, is the most accurate way to estimate how far along you are. If the scan date differs from the calculator, go with the scan.

What to do next

Not sure if you are pregnant? Use the pregnancy test calculator to find the best day to test.
Get your due date from the due date calculator, including IVF and scan dates.
Read about early signs of pregnancy.
Trying to conceive? Try the ovulation calculator.

See your pregnancy week by week.