Due Date Calculator by Last Period

The most common way to work out a due date starts with one date: the first day of your last menstrual period. Add 280 days and you have your estimated due date, the anchor for your whole pregnancy calendar. The due date calculator by last period above does it for you, adjusts for your cycle length, and shows your trimester dates and milestones.

This page explains the method step by step, shows how cycle length changes the answer, and works through examples so you can check the calculator’s result by hand.

Why the Last Period?

Doctors date pregnancy from the last period because it is the date most women know. Conception is harder to pin down — sperm can live for up to five days, ovulation does not always happen on day 14, and many pregnancies begin without anyone noting the exact day. The period, by contrast, is a date you can usually remember or find in an app. The convention counts pregnancy from the period even though conception happens about two weeks later, which is why a pregnancy is called 40 weeks long even though the baby has only been growing for 38.

Naegele’s Rule, Step by Step

The method is called Naegele’s rule, after the German obstetrician who described it in the 1800s. Here is how it works:

1. Find the first day of your last period. Not the last day, and not the heaviest day — the first day bleeding started.
2. Add 280 days. That is 40 weeks, or nine calendar months plus seven days.
3. Adjust for cycle length. If your cycles are longer than 28 days, add the extra days; if shorter, subtract them.
4. The result is your estimated due date. It is the day you reach 40 weeks 0 days.

A shortcut many people use: add seven days to the period date, then count back three months. For a last period of March 15, 2026, add seven days to get March 22, then count back three months to December 22 — close to the exact answer of December 20, but two days out, because months are not all the same length. The calculator uses the exact 280 days, so trust it over the shortcut.

Worked Example: LMP March 15, 2026

First day of the last period: March 15, 2026. Add 280 days: March has 16 days left after the 15th, then April 30, May 31, June 30, July 31, August 31, September 30, October 31, November 30, and 20 days into December — December 20, 2026. That is your due date. Your conception was most likely around March 29, 2026, your first trimester ends June 20, 2026, and your third trimester starts September 27, 2026. The due date calculator above shows all of these dates from the same input.

How Cycle Length Changes the Date

Naegele’s rule assumes a 28-day cycle with ovulation on day 14. If your cycles run longer, you ovulate later, and the due date moves later by the same number of days. The calculator applies this adjustment automatically when you enter your cycle length.

If your cycles vary a lot from month to month, use your average cycle length over the last three to six months. And if your cycles are very irregular, the period-based date is less reliable — an early ultrasound before 14 weeks will give a better date, and your doctor will usually adopt it.

What If You Do Not Remember Your Last Period?

It happens often, especially if your periods are irregular or you were not tracking them. Your options, in order of reliability:

• Check your period-tracking app. Most apps keep months of history, and the first day of the last period is usually there.
• Check your calendar or messages. Many women can narrow it down from travel, events or messages to friends.
• Use the conception date instead. If you know when you conceived, add 266 days. The calculator above has a conception option.
• Get an early ultrasound. A scan before 14 weeks dates the pregnancy to within a few days, and it is the most accurate method available.

Do not guess and then worry about the guess. A wrong period date shifts everything — the due date, the scan windows, the screening tests — so it is worth getting the date right, or letting the scan set it.

Trimester Dates From Your LMP

Once you have your due date, the trimesters follow. The table below shows them for a last period of March 15, 2026.

The first trimester is when the baby’s organs form and when morning sickness is worst. The second is usually the most comfortable — energy returns, the bump shows, and the anatomy scan happens. The third is when the baby grows fastest and when sleep gets harder. Knowing which trimester you are in helps you know what to expect and which appointments are due.

How the Calculator Shows Your Progress

As well as the due date, the calculator shows how far along you are today: your gestational age in weeks and days, your trimester, and a progress bar showing what percentage of the 40 weeks has passed. For a last period of March 15, 2026, on November 1, 2026 you are 33 weeks and 0 days along — 83% of the way through — in your third trimester, with 49 days to go until the due date of December 20. The progress bar fills as the weeks pass, which makes it a satisfying thing to check each week.

When the Period-Based Date Is Wrong

The period method assumes you ovulated on day 14 of a regular cycle. It is wrong when you ovulated early or late, when your cycles are irregular, when you conceived while breastfeeding, or when you stopped hormonal contraception recently. In those cases the scan date wins. If the scan and the period date disagree by more than seven days in the first trimester, your doctor will change the due date to the scan date — and all your milestones move with it. That is normal, not a problem, and it happens in a large minority of pregnancies.

Related tools: your daily water intake in pregnancy and your calorie needs as they rise through the trimesters. The American College of Obstetricians and Gynecologists explains how due dates are set and when they change.

Key Takeaways

Keep the reference figures below handy while you plan.

What to remember

• Your due date is 280 days after the first day of your last period, or 266 days after conception.
• Only about 5 percent of babies arrive on their due date — most arrive within two weeks either side.
• A longer cycle pushes the due date later: a 35-day cycle adds seven days.
• An ultrasound before 14 weeks is the most accurate way to date a pregnancy.
• Full term is 39 to 40 weeks; early term is 37 to 38 weeks.

Your checklist

1. Enter the first day of your last period, or your conception date, in the calculator above.
2. Note your due date, your trimester dates and your milestones.
3. Book your first prenatal appointment before 10 weeks.
4. Ask about the dating scan at 11 to 14 weeks.
5. Have the hospital bag ready by 37 weeks.

Mistakes to avoid

• Counting from the last day of the period instead of the first day.
• Forgetting the cycle-length adjustment when cycles are not 28 days.
• Treating the due date as an appointment — it is an estimate.
• Panicking when the scan changes the date — that is normal and means the dating improved.
• Planning everything for the exact due date with no slack either side.

Frequently Asked Questions

How do I calculate my due date from my last period?

Add 280 days (40 weeks) to the first day of your last menstrual period. If your last period started March 15, 2026, your due date is December 20, 2026. If your cycles are longer or shorter than 28 days, adjust by the difference — a 35-day cycle pushes the date a week later.

What is the first day of my last period?

The first day bleeding started, not the last day of the period and not the heaviest day. If your period started on March 15, that is the date to enter, even if the bleeding lasted until March 20.

Why is pregnancy counted from the last period, not conception?

Because most women know when their period started and do not know exactly when they conceived. Ovulation usually happens about 14 days after the period starts, so the 40-week count includes roughly two weeks before conception. It is a convention, not a biological claim.

My cycles are 35 days. How does that change my due date?

You probably ovulate around day 21 instead of day 14, so your due date moves seven days later than the standard calculation. For a last period of January 1, 2026, the standard date is October 8, but with a 35-day cycle it is October 15. The calculator above applies the adjustment automatically.

What if I do not remember my last period?

Check a period-tracking app, your calendar or old messages. If you know the conception date, add 266 days instead. And an ultrasound before 14 weeks dates the pregnancy to within a few days — it is the most accurate method and your doctor will usually adopt its date.

Can my due date change after the first scan?

Yes, and it often does. If the first-trimester scan disagrees with the period-based date by more than seven days, your doctor will usually change the due date to the scan date. Later scans are less accurate for dating, so the first scan is the one that counts.

Disclaimer: This calculator provides estimates for general information only and is not medical advice. Due dates are estimates and an early ultrasound is the most accurate way to date a pregnancy. Talk to your doctor or midwife about your pregnancy, scans and any concerns.