Conception Date Calculator

Estimate the date you conceived, and the days it most likely happened, from your due date, your last period or your baby's birthday.

This tool gives general estimates only and is not medical advice. Always follow the guidance of your healthcare provider.

How the conception date is estimated

When a pregnancy is dated, it is assumed to begin two weeks after the first day of your last period, because that is when ovulation usually happens in a 28-day cycle. A due date is 40 weeks (280 days) after the last period, so conception is estimated at 266 days, or 38 weeks, before the due date.

The calculator works from whichever date you know:

Due date: it counts back 266 days. This is the best choice if your due date came from an ultrasound.
Last period: it adds 14 days, adjusted if your cycle is longer or shorter than 28 days.
Baby's date of birth: it counts back from the birth, using how many weeks pregnant you were when your baby arrived.

Why the answer is a range

Conception is the moment an egg is fertilized, and that can only happen within about a day of ovulation. Sperm can survive in the body for up to about five days, so the intercourse that led to the pregnancy may have happened on any of the five days before conception, or on the day itself. That is why the result shows a six-day window as well as a single date.

The date itself is also an estimate. Ovulation does not always happen on the expected day, and cycles vary from month to month, so the real conception date can be several days earlier or later than the one shown.

Which starting date is most accurate?

A due date from a first-trimester ultrasound is the most reliable starting point, because the scan measures the baby directly. A date based on your last period assumes a regular cycle with ovulation at the usual time. If your cycles are irregular, or you are not sure when your last period started, use your scan-based due date if you have one.

Conception after IVF

With IVF the date is known rather than estimated. Fertilization happens in the lab, so it is the embryo transfer date minus the age of the embryo: three days earlier for a 3-day embryo, or five days earlier for a 5-day embryo (blastocyst). Your clinic will use these dates to set your due date.

Frequently asked questions

How many days before my due date did I conceive?

About 266 days, or 38 weeks. A due date is set at 280 days after the first day of your last period, and conception is assumed to happen 14 days into that count.

How accurate is the conception date?

It is an estimate that can be off by several days. It is most accurate when it is based on a due date from an early ultrasound, and least accurate when cycles are irregular.

Can this tell me who the father is?

No. The estimate is not precise enough for that, and the likely window covers several days. Only a DNA test can establish paternity.

Is the conception date the same as the date I had sex?

Not always. Sperm can survive for up to about five days, so conception can happen several days after intercourse. That is why the result shows a window of days.

Why does my pregnancy count start before I conceived?

Pregnancy is counted from the first day of your last period, about two weeks before conception. So at the moment you conceive, you are already counted as two weeks pregnant.