Pregnancy tool

Conception Date Calculator

Find the date you most likely conceived — either from the first day of your last period, or by counting back from a due date you already have — and see the week your pregnancy is dated from.

Enter your details

What do you already know? My last period Use the first day of your last period and your usual cycle length. My due date Count back 266 days from an estimated due date you were given.

First day of your last period (LMP) Day 1 of your most recent menstrual period, not the last day.

Cycle length (days) From day 1 of one period to day 1 of the next. 20–45 days.

Results update automatically as you change the dates. Everything is calculated in your browser.

Estimated conception date

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Possible window

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Pregnancy week 0 (LMP)

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Pregnant on that day

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Estimated due date

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Key dates

Conception estimate with the week-zero date, gestational age and due date
Milestone	Calendar date

How this conception date calculator works

There are two ways to reach the same estimate, and this page uses whichever one your input supports. These are the exact formulas.

From your last period: conception = LMP + (cycle length − 14)
From your due date: conception = due date − 266 days
Possible window = conception − 5 days → conception + 1 day
Week 0 (LMP) = conception − (cycle length − 14), or due date − 280 days
Due date from an LMP = LMP + 280 days + (cycle length − 28)

Default assumption. The calculator assumes a 28-day cycle with ovulation on day 14 and a luteal phase of 14 days, which is the textbook basis for dating a pregnancy from the last period.
Why 266 days? A pregnancy measured from conception is about 38 weeks, or 266 days. The familiar 280-day (40-week) count starts about two weeks earlier, at the last period.
Cycle adjustment. With a 35-day cycle, ovulation is placed on cycle day 21 (35 − 14), so conception moves seven days later than a 28-day cycle would suggest.
Conception is a range, not a moment. The seven-day window reflects how long sperm can survive, so intercourse earlier in the week can still be the one that led to pregnancy.
All date maths uses whole UTC calendar days, so daylight-saving changes can never shift a result by a day.

Dates that do not exist, such as 30 February, and dates that would place conception in the future are rejected instead of being quietly adjusted.

What "pregnancy week 0" means

Antenatal care dates a pregnancy from the first day of your last period, not from conception. That means you are already counted as about two weeks pregnant on the day you conceive — a convention, not a mistake. It exists because the LMP is usually easier to remember than ovulation.

So the same day can be described in two ways: it is the conception date, and it is also roughly gestational week 2, day 0. Every scan measurement, every "12-week" milestone and every due date you are given follow the LMP convention, which is why this page shows both numbers side by side.

If you entered a due date instead of a period, the calculator reverses the same rule: the estimated LMP is your due date minus 280 days, and conception is that date plus roughly two weeks.

When to talk to a clinician

Contact a midwife, doctor or sexual health clinic if any of the following applies:

You are unsure of your last period, or your cycles are irregular, very short or very long — a dating ultrasound can be more accurate than any calculation.
You have a positive pregnancy test and any pain, bleeding or dizziness.
You are taking medication, have a long-term condition, or want advice about early pregnancy care.
You need contraception or emergency contraception — this page cannot help with either.
You are trying to conceive and want to understand your cycle in more detail.

Not medical advice. Baby Tools is an educational calculator. It cannot confirm a pregnancy, date it reliably, or tell you who the biological parent is. Always rely on a clinician for medical decisions.

Frequently asked questions

How is the conception date calculated?

From a known last period, conception is estimated as the LMP plus your cycle length minus 14 days. From a known due date, conception is estimated as the due date minus 266 days, because a pregnancy counted from conception lasts about 38 weeks.

Why is conception not 280 days before my due date?

A due date is counted from the last period, which is about two weeks before conception. The 280-day count therefore includes those two weeks, while a count that starts at conception is 266 days.

How wide is the possible conception window?

This calculator shows a seven-day window: the five days before the estimated conception date, the day itself and the day after. Sperm can survive for several days, so the actual date may fall anywhere in that range.

Is the conception date exact?

No. It is an estimate based on average cycle timing or on a due date that may itself have been revised after an ultrasound. Even with an early scan the answer stays a range rather than a single guaranteed day.

Sources and disclaimer

Primary sources used for the method and the notes on this page:

NHS — National Health Service (UK), including its guidance on how pregnancy is dated and on antenatal appointments.
ACOG — The American College of Obstetricians and Gynecologists, for its patient information on estimating gestational age and dating a pregnancy.

This page is informational only and is not a medical device. It does not create a clinician–patient relationship and must not be used to make medical decisions. Your dates are processed only in this browser tab; nothing is uploaded, stored or shared.

More baby tools

Start from the Baby Tools home page, or try the due date calculator for a full pregnancy timeline, the ovulation calculator to find your fertile window, and the implantation calculator to estimate when implantation may happen.