How to Calculate Your Due Date From Your Last Period

Published November 8, 2026 · Updated November 8, 2026 · 7 min read · by the Sakura5 team

To calculate your due date, add 280 days (40 weeks) to the first day of your last menstrual period. A quick way to do it by hand is Naegele's rule: take the first day of your last period, add 7 days, subtract 3 months, and add 1 year. If your last period started on June 3, 2026, your estimated due date is March 10, 2027.

That date is an estimate, and it is meant to be. The NHS notes that pregnancy normally lasts from 37 to 42 weeks from the first day of your last period, so a healthy baby can arrive weeks either side of the date on paper. Your due date is a planning anchor, not a deadline.

This guide explains how the calculation works, how to adjust it for longer or shorter cycles, how it changes if you know your conception date or conceived through IVF, and why your first ultrasound may move the date.

Why pregnancy is counted from your last period

It surprises many people that pregnancy weeks are counted from the first day of the last period, not from conception. The reason is practical: most people know when their period started, but very few know the exact day they ovulated or conceived.

In a typical 28-day cycle, ovulation happens around two weeks after the period starts. So by the time conception happens, you are already counted as about 2 weeks pregnant, and a missed period usually falls at about 4 weeks. This is also why "40 weeks of pregnancy" includes roughly two weeks before you were actually pregnant.

The American College of Obstetricians and Gynecologists (ACOG) recommends that the gestational age and estimated due date be worked out as soon as the last menstrual period, the first accurate ultrasound, or both are available, and that changes to the due date afterwards be reserved for rare circumstances. That guidance is in ACOG's committee opinion on methods for estimating the due date.

How to calculate your due date from your last period

1. Find the first day of your last period

Use the first day of full bleeding, not spotting. If you track your cycle, this date is already in your records. If you are not sure, write down your best estimate and mention the uncertainty to your midwife or doctor.

2. Add 280 days

Count 280 days forward, or use Naegele's rule: add 7 days, subtract 3 months, add 1 year. Both give the same result for a 28-day cycle.

3. Adjust for your cycle length

The standard calculation assumes a 28-day cycle with ovulation around day 14. If your cycles are reliably longer or shorter, ovulation likely happened later or earlier. A common adjustment is to add the number of days your cycle is longer than 28, or subtract the days it is shorter. The NHS due date calculator lets you enter your cycle length for this reason. If your cycles are irregular, the period-based date is less reliable and an early ultrasound becomes more important.

4. Work out how many weeks pregnant you are

Count the days from the first day of your last period to today and divide by 7. The whole number is the completed weeks, and the remainder is the extra days. Clinicians write this as weeks plus days, such as 8+3 or 8 weeks 3 days.

A worked example

Lan's last period started on Wednesday, June 3, 2026. Her cycles are usually 28 days.

• Naegele's rule: June 3 plus 7 days is June 10. Minus 3 months is March 10. Plus 1 year gives an estimated due date of March 10, 2027.
• Check with days: June 3, 2026 plus 280 days is also March 10, 2027.
• How far along on August 1, 2026: 59 days since June 3, which is 8 weeks and 3 days, written 8+3.
• If her cycles were 31 days: adding the 3 extra days gives an estimate of about March 13, 2027.

At her first scan, the measurement of the baby suggests a date very close to March 10, so the due date stays as calculated. If the scan had pointed to a noticeably different date, her clinician might have changed it, as explained below.

Once you know your due date, following the weeks as they pass is easier with a tracker that counts for you. Petala, a cycle and pregnancy tracker for iPhone, now free to download on the App Store, follows all 42 weeks with what is happening each week and playful baby-size comparisons, and keeps your pregnancy details on your iPhone rather than in an online account.

Other ways to calculate a due date

From a known conception or ovulation date

If you know when you ovulated or conceived, for example from ovulation tests, add 266 days (38 weeks) to that date. This gives the same result as the period method for someone who ovulated on day 14, and can be more accurate if your ovulation was earlier or later than usual.

After IVF

With IVF, the timing is known precisely, so the due date is calculated from the embryo transfer date and the embryo's age. ACOG's guidance uses 266 days minus the embryo's age at transfer, which works out to 261 days after a day-5 transfer and 263 days after a day-3 transfer. Your fertility clinic will usually give you this date.

From an early ultrasound

In early pregnancy, the baby's length (crown-rump length) is a very accurate guide to gestational age. That is why the dating scan carries so much weight. The NHS notes that the 12-week scan can estimate more accurately how many weeks pregnant you are.

Why your due date might change after your first scan

ACOG's guidance is that ultrasound measurement in the first trimester is the most accurate way to confirm or set gestational age. If the dating scan and your period-based date differ by more than a set margin, your clinician will usually go with the ultrasound date. The margin depends on how early the scan is done, because measurement is most precise very early in pregnancy.

ACOG also recommends that once the due date is set from the last period and the first accurate ultrasound, it should only be changed in rare circumstances. So if your date moves, it typically moves once, early, and then stays put. A pregnancy without an ultrasound confirming or revising the due date before 22 weeks is considered less precisely dated.

If you are tracking your pregnancy in an app such as Petala, update it to match the date your midwife or doctor gives you. Their date is the one your care is planned around.

What a due date does and does not tell you

A due date tells you roughly when to expect your baby and anchors the timing of scans, tests, and appointments. It does not predict the day labor will start. Babies born between 37 and 42 weeks are typically considered within the normal range, and relatively few arrive on the exact date.

If you are not yet pregnant and are trying to conceive, our guide on when to take a pregnancy test covers the step before this one.

FAQ

How is a due date calculated?

Most due dates are calculated by adding 280 days, or 40 weeks, to the first day of your last menstrual period. Naegele's rule is a shortcut for this: add 7 days, subtract 3 months, and add 1 year. An early ultrasound may then confirm or adjust the date.

How accurate is a due date?

It is an estimate. The NHS notes that pregnancy normally lasts 37 to 42 weeks from the first day of the last period, so birth anywhere in that range is common. Dates confirmed by a first-trimester ultrasound are generally more reliable than those based on the last period alone.

Can I calculate my due date from conception?

Yes. Add 266 days, or 38 weeks, to the date of conception or ovulation. This is useful if you tracked ovulation or know the exact date, but most people do not, which is why the last-period method is the standard. For IVF pregnancies, the clinic calculates the date from the transfer.

Why did my doctor change my due date?

Usually because the first ultrasound measurement suggested a different gestational age than your last period did, by more than the margin clinicians use. Early ultrasound is very accurate for dating, so ACOG recommends going with it in that case, and avoiding further changes later in pregnancy except in rare circumstances.

This article is general information, not medical advice. Your midwife, doctor, or obstetrician sets your official due date, so always follow the date and care plan they give you. Contact them with any questions or concerns about your pregnancy.

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