Estimated Due Date Calculator

Your estimated due date — the EDD — is the single date your whole pregnancy revolves around. It is the day you reach 40 weeks, and every scan, test and appointment is scheduled relative to it. The estimated due date calculator above works it out from your last period or your conception date, and shows your trimester dates, your milestones and how far along you are today.

This page explains what the EDD is, how it is calculated, how accurate it is, and what happens when it changes — because for many pregnancies, it does.

What Is an Estimated Due Date?

An estimated due date is the date your pregnancy is expected to reach 40 weeks. The word “estimated” matters: it is a prediction, not a promise. Only about 5% of babies arrive on their EDD, and about 80% arrive within two weeks either side of it. Doctors use the EDD to schedule care — the dating scan at 11 to 14 weeks, the anatomy scan at 18 to 22 weeks, glucose screening at 24 to 28 weeks — and to decide when a pregnancy is early, full term, late or postterm. Without it, there would be no calendar at all.

How the EDD Is Calculated

There are two ways, and the calculator offers both. From the last period: add 280 days to the first day of your last menstrual period. From conception: add 266 days to the conception date. Both should give the same date when the cycle is regular. For a last period of March 15, 2026, the EDD is December 20, 2026. For a conception date of March 29, 2026, the EDD is also December 20, 2026 — 266 days later. The two methods agree because conception happens about 14 days after the period starts, and 280 minus 14 is 266.

Worked Example: LMP January 1, 2026

Last period January 1, 2026, 28-day cycle: add 280 days and the EDD is October 8, 2026. Conception was most likely around January 15, 2026, and 266 days from January 15 is also October 8, 2026. The first trimester ends April 8, 2026, the second ends September 26, 2026, and the third starts September 27, 2026. The calculator above shows all of these dates from a single input.

EDD by Method Compared

An early ultrasound is the most accurate method of all. Before 14 weeks, babies grow at a very consistent rate, so measuring the baby dates the pregnancy to within three to five days. After 20 weeks, growth rates vary, and the same measurement can be off by two to three weeks. That is why the dating scan matters so much — it is the one chance to set the date precisely, and once set, it is rarely changed.

Why Your EDD Might Change

It is common for the EDD to move after the first scan. If the scan disagrees with the period-based date by more than seven days, most doctors will adopt the scan date, and everything moves with it: the trimester boundaries, the scan windows and the screening tests. Other reasons the date changes include irregular cycles, ovulation that happened early or late, conception while breastfeeding, or a period date that was misremembered. A changed EDD is not a sign that anything is wrong — it is a sign that the dating has improved.

Your EDD and Your Trimesters

The EDD anchors the trimesters. For a last period of March 15, 2026 and an EDD of December 20, 2026:

What Happens Around Your EDD

As the EDD approaches, appointments become weekly. Your provider checks the baby’s position, your blood pressure, and signs of labour. If you pass the EDD, monitoring increases: most providers offer extra checks at 41 weeks and discuss induction. The evidence supports waiting until 41 weeks for most low-risk pregnancies, with induction usually recommended by 42 weeks, when the risks of continuing begin to outweigh the benefits of waiting. Only about 5% of babies arrive on the EDD itself, so passing it by a few days is normal, not a cause for alarm.

• Before 37 weeks: preterm. The baby may need special care, and the earlier the birth, the more support is needed.
• 37 to 38 weeks: early term. The baby is ready, but the brain and lungs are still maturing.
• 39 to 40 weeks: full term. The ideal window, with the best outcomes for mother and baby.
• 41 weeks: late term. Monitoring increases and induction is discussed.
• 42 weeks and beyond: postterm. Induction is normally recommended.

Planning Around Your EDD

Use the EDD to plan, but plan with flexibility. Maternity leave, childcare, the hospital bag and the nursery should all be ready by 37 weeks, because one baby in ten arrives before then. Keep two weeks of slack either side of the date for anything that cannot move. And remember that the EDD is a medical tool, not a deadline: your baby will arrive when ready, and the date is there to guide care, not to create anxiety. If you are counting down the days, check your daily water intake — hydration matters more in late pregnancy — and your calorie needs, which are about 450 a day above normal in the third trimester.

1. Work out your EDD with the calculator above.
2. Book your first prenatal appointment before 10 weeks.
3. Ask about the dating scan at 11 to 14 weeks.
4. Use the EDD to plan leave and childcare, with two weeks of slack.
5. Have the hospital bag ready by 37 weeks.
6. If the scan changes your EDD, update your plans — the new date replaces the old one entirely.

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

At a Glance

Use the due date calculator above with your own numbers, and 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

What is my estimated due date?

Add 280 days to the first day of your last period, or 266 days to your conception date. For a last period of March 15, 2026, the EDD is December 20, 2026. The calculator above works it out from either date, with your trimester dates and milestones.

How accurate is an estimated due date?

It is an estimate. Only about 5% of babies arrive on their EDD, and about 80% arrive within two weeks either side. An ultrasound before 14 weeks is the most accurate dating method, correct to within a few days.

What is the difference between EDD and due date?

Nothing — EDD is short for estimated due date, and the two terms mean the same thing. Both refer to the date your pregnancy reaches 40 weeks from the first day of your last period.

Can my estimated due date change?

Yes, and it often does after the first scan. If the scan disagrees with the period-based date by more than seven days, your doctor will usually adopt the scan date. Irregular cycles and uncertain period dates are the most common reasons.

What happens if I go past my due date?

Most providers offer extra monitoring at 41 weeks and discuss induction. Waiting until 41 weeks is supported by the evidence for low-risk pregnancies, and induction is usually recommended by 42 weeks. Passing the date by a few days is normal.

Is the due date from conception or from my last period?

Both methods exist and should agree. The last-period method adds 280 days; the conception method adds 266 days. The 14-day difference is the gap between the period and ovulation. Use whichever date you know more reliably.

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.