Pregnancy Due Date Calculator

A due date is an estimate of when you would reach 40 weeks of pregnancy. It gives your care team a reference point for scans, tests and appointments rather than a prediction of the day you will give birth.

Choose the dating method that matches what you know, and this calculator will estimate your due date along with your conception date, how far along you are today, and the trimester you are in.

These calculators provide estimates for informational purposes only. They are not intended to diagnose, treat, prevent, or replace professional medical advice. Pregnancy and menstrual cycles can vary, so results may not be exact.

How this calculator works

What you need to enter:

The first day of your last menstrual period, or a known conception or IVF transfer date
Your average cycle length, if you are dating from your last period

Dating from your last period

The standard method, known as Naegele's rule, adds 280 days (40 weeks) to the first day of your last menstrual period. That first day is used because it is something most people can recall, whereas the exact moment of conception usually is not.

It is worth knowing what this implies: pregnancy is counted from about two weeks before you actually conceived. At the moment of a positive test you are typically already described as four weeks pregnant.

Adjusting for your cycle length

The 280-day rule assumes a 28-day cycle with ovulation on day 14. If your cycles run longer, you ovulate later, so the pregnancy starts later relative to your last period and the due date moves back by the difference.

This calculator applies that shift for you. On a 35-day cycle it adds seven days; on a 24-day cycle it subtracts four. The adjustment only affects the follicular phase before ovulation, because the span from conception to birth stays around 266 days either way.

Dating from conception or an IVF transfer

If you know the date of conception, or tracked ovulation and know it closely, the calculator adds 266 days instead. This skips the cycle-length assumption entirely and is usually the more precise route.

IVF dating is more precise again, because the age of the embryo at transfer is known exactly. Choose the IVF option and select your transfer day, and the calculation counts from the embryo's true age rather than from a cycle estimate.

A worked example

Suppose the first day of your last period was 26 June 2026 and your cycles average 31 days.

Start from 26 June 2026, the first day of the last period.
Add 280 days, which lands on 2 April 2027 for an average 28-day cycle.
Your cycle is 3 days longer than 28, so add 3 more days.
The estimated due date becomes 5 April 2027, with conception estimated around 13 July 2026.

Most births happen in the weeks either side of this date rather than on it. Fewer than one in twenty babies arrive on their estimated due date.

What this calculator can't tell you

The result is an estimate. It assumes ovulation happened at a typical point in your cycle, which is not always the case.
If you are unsure of the first day of your last period, or your cycles vary a lot, the estimate becomes less dependable.
An early ultrasound is generally the most accurate way to date a pregnancy. If a scan gives a different date, your care team will usually use the scan.
Due dates are commonly revised after a first-trimester scan. A change of a few days is routine and not a cause for concern.

What to do next

Check how far along you are today with the pregnancy week calculator.
See the full trimester-by-trimester timeline with the pregnancy calculator.
Bring the date to your first appointment, where a scan may refine it.

Common questions

How accurate is a due date calculator?

It is a well-established estimate, not a measurement. The calculation assumes a regular cycle and ovulation around day 14, so it is most dependable when your cycles are regular and you are sure of your last period. An ultrasound in the first trimester dates a pregnancy more accurately, and around half of estimated due dates are adjusted after that scan.

Why does pregnancy count from my last period if I was not pregnant yet?

Because the first day of a period is a date most people can identify, while ovulation and conception usually pass unnoticed. Dating from the last period gives everyone a consistent reference. It is the reason a pregnancy is described as 40 weeks even though it lasts about 38 weeks from conception.

What if my cycle is not 28 days?

Enter your real average cycle length. The calculator shifts the due date by the difference from 28 days, because a longer cycle means later ovulation and therefore a later due date. If your cycles vary widely from month to month, treat the result as a rough range rather than a single day.

Can my due date change?

Yes, and it often does. If an early ultrasound measures the pregnancy as further along or less far along than your dates suggest, your care team may revise the due date. Later scans are less reliable for dating, so a date set in the first trimester is usually kept.

Will my baby be born on the due date?

Rarely. A due date marks 40 weeks, but birth anywhere from 37 to 42 weeks is common. Think of it as the middle of a likely range rather than an appointment.

Do you store the dates I enter?

No. The calculation runs in your browser and nothing you type is sent to a server or saved anywhere.