Pregnancy Due Date Calculator

What this calculator does

Find out when your baby is due and how far along you are. This calculator estimates your due date, how many weeks and days pregnant you are, and your trimester. It also gives the dates of key milestones, such as the start of each trimester and of full term.

A due date is set 40 weeks (280 days) after the first day of your last period. Conception usually happens about two weeks after that period starts. But pregnancies are still counted from the period, not from conception. So when people say "weeks pregnant", they mean weeks since the period began.

You can start from whichever date you know best. That could be the first day of your last period (with your usual cycle length), the conception date, an IVF embryo transfer date, or an ultrasound scan. The calculator shows how far along you are on any date you pick, the trimester, and a dated timeline. It's an estimate, not a diagnosis. Your doctor or midwife may set a different date, often from an early ultrasound, and theirs is the one to follow.

Where it's used

Why this math matters outside the classroom:

• Naegele's rule. A last period that began January 10 plus 280 days gives an estimated due date of October 17 in a non-leap year.
• Weeks pregnant. Counting 84 days since the last period gives 12 weeks 0 days, since each week is 7 days.
• IVF dates. For a 5-day embryo transfer, adding 261 days to the transfer date gives the estimated due date.
• Planning ahead. Expecting parents plan leave and childcare start dates around the estimated due date, updating it as their doctor advises.
• Trimester milestones. Families mark the start of the second trimester at 14 completed weeks and the third at 28 on a shared calendar.

How to use it

1. Choose what to start from: your last period, a known conception date, an IVF transfer, or an ultrasound.
2. Enter that date. For the last-period method, also enter your average cycle length. For IVF, choose a 3-day or 5-day embryo. For an ultrasound, enter the weeks and days shown on the scan report.
3. Leave the second date as today to see how far along you are now, or change it to check any other day.
4. Read your estimated due date, how far along you are in weeks and days, your trimester, progress and days left. Then check the milestone table for the dates ahead.

What the inputs mean

First day of last period

The first day of bleeding of your most recent period, not the last day.

Average cycle length

The number of days from the start of one period to the start of the next. The standard rule assumes 28. A longer cycle usually means you ovulate later, so the due date moves later by the same number of days. A shorter cycle moves it earlier.

Conception date

Use this only if you know the date fairly exactly, for example from tracking ovulation.

Embryo transfer date

The date the embryo was transferred. A 3-day embryo was fertilized 3 days earlier. A 5-day embryo (called a blastocyst) was fertilized 5 days earlier.

Ultrasound date

The date of the ultrasound, usually an early dating scan.

Gestational age at the scan: weeks

How far along the scan said you were (the gestational age), in whole weeks. Enter the extra days in the next box. For 8 weeks 3 days, enter 8 and 3.

Gestational age on this date

The date to show how far along you are and to count down to milestones from. It starts as today.

Formula and method

From last period:  due date = LMP + 280 days + (cycle length − 28) days
From conception:   due date = conception date + 266 days
From IVF transfer: due date = transfer date + 263 days (3-day embryo) or + 261 days (5-day embryo)
From ultrasound:   due date = scan date + 280 days − (weeks × 7 + days at the scan)
Gestational age = as-of date − (due date − 280 days), shown as weeks + days
Progress = gestational age in days ÷ 280 × 100

In the formulas, LMP means the first day of your last menstrual period. Gestational age is how far along the pregnancy is, counted in weeks and days. The last-period method is called Naegele's rule, with a change for cycles longer or shorter than 28 days.

The conception method uses 266 days. That's 280 minus the 14 days from the start of a typical 28-day cycle to ovulation. For IVF, the embryo's age at transfer is also taken off 266.

Gestational age is always counted from the date 280 days before the due date. That matches the way doctors date pregnancies. Trimesters follow the usual split: the first runs to 13 weeks 6 days, the second to 27 weeks 6 days, and the third starts at 28 weeks. Dates are counted as whole calendar days, so time zones and daylight saving don't affect them.

The ultrasound method counts back from the scan by the age the scan measured, then forward 280 days. The milestone table uses the term dates from the American College of Obstetricians and Gynecologists. Early term starts at 37 weeks 0 days, full term at 39 weeks 0 days, late term at 41 weeks 0 days, and post-term at 42 weeks 0 days. The term window shown runs from 37 weeks 0 days to 41 weeks 6 days.

Worked example

The first day of the last period was January 1, 2026, with a 28-day cycle. How far along is the pregnancy on April 1, 2026?

1. Days to add: 280 + (28 − 28) = 280.
2. Jan 1, 2026 + 280 days = Thursday, Oct 8, 2026.
3. Gestational age on Apr 1, 2026: 90 days since Jan 1, which is 12 weeks, 6 days.
4. 12 weeks 6 days is before 14 weeks, so it's the first trimester. There are 190 days left until the due date.
5. Progress: 90 ÷ 280 × 100 = 32%.
6. Milestones: the second trimester begins Thursday, Apr 9, 2026 (in 8 days). Full term begins Thursday, Oct 1, 2026. The term window runs from Sep 17, 2026 to Oct 21, 2026.

Result: The estimated due date is Thursday, Oct 8, 2026. On Apr 1, 2026, the pregnancy is 12 weeks, 6 days along: first trimester, and 32% of the way.

Understanding the result

A due date marks the middle of a window, not a deadline. Many babies are born in the weeks either side of it, and few arrive on the exact date.

If this date differs from the one your healthcare provider gives you, use theirs. Early ultrasound measurements are generally seen as more reliable than a date from your last period, especially when cycles are irregular.

For an IVF pregnancy, the transfer date is known exactly, so this estimate is usually the most consistent of the methods. If you have both a last-period date and an early ultrasound, compare the two. When they differ, your provider decides which date to use.

Assumptions and limitations

• For the last-period method, you ovulate about 14 days before your next period would start. So each extra day in your cycle moves the due date one day later.
• Pregnancy lasts 280 days from the matching first day of the last period.
• Dates are whole calendar days. No time of day is involved.
• It can't allow for irregular cycles, being unsure of your period date, or bleeding in early pregnancy that might be mistaken for a period.
• It doesn't predict when labor will actually start, and it doesn't replace prenatal care.
• Naegele's rule dates from the early 1800s and was built around a regular 28-day cycle. The cycle adjustment helps, but it's still a rule based on averages.
• It uses the same rules for every pregnancy, including twins or more. It doesn't account for any medical factor. Only a healthcare professional can assess those.

Frequently asked questions

How this calculator is checked

Method: Naegele's rule (LMP + 280 days) with cycle-length adjustment; conception + 266 days; IVF transfer + 266 − embryo age (263 or 261 days); ultrasound date + 280 days − gestational age at the scan; common trimester boundaries at 14 and 28 completed weeks; ACOG term definitions (Committee Opinion 579, 2013) for the milestone table. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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