Due Date Calculator

How to use this tool

Choose how you want to work it out, then enter the date. Most people start from the first day of their last period and their usual cycle length. If you know when you conceived, had an IVF embryo transfer or have had an early ultrasound, choose that instead, since those are more precise. The due date appears at the top, with how far along you are today, the trimester dates, the term windows and a timeline you can print or save.

The date people most often get wrong is the period date. It is the first day of bleeding in the last period, not the day it ended and not the day you think you conceived. Using the end of the period moves the due date several days too late.

What the calculation is doing

A pregnancy is counted in weeks from the first day of the last period, a date most people know, rather than from conception, which usually cannot be pinned down. With a 28 day cycle, ovulation and conception happen around day 14, and birth comes on average about 38 weeks after that. Adding the two weeks before conception gives the familiar 40 weeks, 280 days.

For a longer cycle, ovulation comes later, so the due date moves later by the same number of days; a 35 day cycle adds a week. From conception the count is 266 days. For IVF the age of the embryo at transfer is known exactly, so the American College of Obstetricians and Gynecologists uses 263 days after a day 3 transfer and 261 days after a day 5 transfer. From an ultrasound, the measured age is taken as correct and the remaining days to 40 weeks are added to the scan date.

Why the due date is a window, not a day

The 280 day rule describes an average. Cycles vary, ovulation does not always fall on day 14 even in a regular cycle, and healthy pregnancies run a range of lengths. The due date is best read as the middle of the term window: from 37 weeks a baby counts as early term, from 39 weeks full term, and the window runs to 41 weeks 6 days before a pregnancy counts as postterm.

That is also why an early scan matters. Up to 13 weeks 6 days, babies grow at a very similar rate, so measuring the embryo gives its age to within about 5 to 7 days. If that disagrees with the period date by more than 7 days, the scan date is normally used. Later in pregnancy babies differ more in size, so later scans are poor at dating and a date set early is kept.

Naegele's rule, the shortcut taught for two centuries, adds 7 days to the period date, takes away 3 months and adds a year. It gives the same answer as adding 280 days for some months but can be up to three days out for others, because months have different lengths. The table below shows both.

Due dates for each month

For a last period starting on the first of each month in 2026, with a 28 day cycle.

A worked example you can check on paper

Take a last period that began on January 1, 2026, with a 28 day cycle. Adding 280 days: the rest of January is 30 days, then February 28, March 31, April 30, May 31, June 30, July 31, August 31 and September 30 bring the total to 272 by September 30, and 8 more days lands on Thursday, October 8, 2026. Naegele's rule agrees here: January 8, back 3 months to October 8, plus a year.

Week 0 is January 1, so conception was most likely around January 15, the first trimester ends at 13 weeks 6 days on April 8, the third trimester begins at 28 weeks on July 16, and full term starts at 39 weeks on October 1. With a 32 day cycle the whole calendar shifts 4 days later, to a due date of October 12.

Mistakes that catch people out

Using the last day of the period. The count starts on the first day of bleeding. The end date adds several days to the estimate.

Putting the conception date in the period box. That makes the due date two weeks too late. Choose the conception method instead.

Ignoring a long or short cycle. A 35 day cycle usually means ovulation a week later than the 28 day assumption, so the due date moves a week.

Mixing up weeks pregnant and the baby's age. Weeks of pregnancy include the two weeks before conception, so a pregnancy at 10 weeks has been developing for about 8 weeks since conception.

Expecting trimester dates to match every source. Books and apps split the trimesters a week apart from each other. This calculator ends the first trimester at 13 weeks 6 days and starts the third at 28 weeks.

Popular calculations

Each opens the calculator with those values filled in, and they can be changed once it is open.

From the last period, 28 day cycle

Longer and shorter cycles

From the conception date

IVF embryo transfer

From an ultrasound

Working back from a due date

How this was checked

The day counts follow the American College of Obstetricians and Gynecologists guidance on estimating the due date: 280 days from the last period, 263 days from a day 3 embryo transfer and 261 days from a day 5 transfer, with the term definitions from its opinion on term pregnancy. As a cross check, one pregnancy entered four ways, a period on January 1, 2026, conception on January 15, a day 5 transfer on January 20 and a scan on February 26 measuring 8 weeks 0 days, gives the same due date, October 8, 2026, every time. Dates are handled as calendar days, so leap years and daylight saving changes do not shift the result.

Every address in the lists above was then opened and checked for a distinct title, the right due date, a timeline that renders, and clean CSV, image and print output containing only the values entered, at desktop, tablet and phone widths.

Common questions

How is a due date calculated?

The standard method adds 280 days, 40 weeks, to the first day of the last menstrual period. That assumes a 28 day cycle with ovulation on day 14, so a longer or shorter cycle moves the date by the difference. From a known conception date it is 266 days, and for IVF it is 263 days after a day 3 embryo transfer or 261 days after a day 5 transfer.

How accurate is a due date?

It is an estimate of the middle of the window when birth is likely, not a prediction of the day. Most babies are born in the weeks around it. An ultrasound in the first trimester, up to 13 weeks 6 days, is the most accurate way to date a pregnancy, and IVF dates are the most precise of all because the embryo's age is known.

Can my due date change?

Yes, usually once, early on. If a first trimester ultrasound differs from the date based on your last period by more than 7 days, the due date is normally changed to match the scan. After that it stays fixed, even if later scans suggest the baby is measuring larger or smaller.

How many weeks pregnant am I?

Count the weeks since the first day of your last period, or since the week 0 date shown by the calculator if you used another method. At two weeks you are only around conception, so the weeks of pregnancy run about two weeks ahead of the age of the embryo.

What counts as full term?

Full term is 39 weeks 0 days to 40 weeks 6 days. Early term is 37 weeks 0 days to 38 weeks 6 days, late term 41 weeks 0 days to 41 weeks 6 days, and postterm 42 weeks or later, the definitions used by the American College of Obstetricians and Gynecologists.

Is this medical advice?

No. It is a planning estimate using the standard formulas. Your midwife or doctor will set the due date in your records, usually from an early ultrasound, and that is the date to use for appointments and tests.

Related Calculators and Reference

• Date Duration Calculator: count the days or weeks between any two dates.
• BMI Calculator: body mass index before pregnancy, which guidance on weight gain is based on.
• Calories Burned Calculator: energy used by walking, swimming and other gentle exercise.
• Time Zone Converter: share the news with family in other countries at a sensible hour.
• Ovulation Calculator: the ovulation day, fertile window and next period from a last period date and cycle length.
• Pregnancy Weight Gain Calculator: the recommended weight gain for the weeks you have reached.
• ACOG Committee Opinion 700: Methods for Estimating the Due Date: the clinical guidance behind the day counts.