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Due date calculator

Estimate your baby’s due date from the first day of your last period (adjusted for your cycle length), your conception date or an IVF transfer date, and see how many weeks along you are and when each trimester starts.

Calculate from

Cycle length is from the first day of one period to the first day of the next. Not sure, or your cycles vary a lot? Leave 28: an early dating scan will give a better date.

Estimated due date
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Enter the first day of your last period.

An estimate, not medical advice. Your doctor or midwife sets your due date, usually with an early ultrasound, and that’s the one to go by. Babies come in their own time: the due date is the middle of a window of several weeks, not a deadline.

  • Free, no sign-up
  • Works on phone and desktop

How to calculate your due date

  1. 1

    Choose what to calculate from: your last period, a conception date, an IVF transfer or a dating ultrasound.

  2. 2

    Enter the date. For a period, add your usual cycle length; for IVF, the embryo’s age; for a scan, how far along it measured.

  3. 3

    Read your estimated due date, how many weeks along you are today, and when each trimester and term window starts.

  4. 4

    Optional: copy the result as text, or add the due date to your calendar. Nothing is saved or sent anywhere.

Features

  • Four ways to date

    Last period with your cycle length, conception, IVF transfers of day-3, day-5 or day-6 embryos, and dating scans.

  • Trimesters and term

    Weeks and days along today, trimester dates and the early, full and late term windows, by ACOG’s definitions.

  • ACOG’s scan rule

    Enter a scan and your period date to see whether ACOG’s guidance would keep your period date or use the scan’s.

  • Private by design

    Your dates never go in the page address and are never stored or sent. There is no share link, on purpose.

How the due date is calculated

Every method lands on the same idea: a pregnancy is counted as 280 days from the first day of the last menstrual period, about 266 days from conception. The calculator follows ACOG Committee Opinion 700, the guidance obstetricians in the US use, written with the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine.

  • Last period: due date = first day of the last period + 280 days + (cycle length − 28 days).
  • Conception: due date = conception (ovulation) date + 266 days.
  • IVF: due date = transfer date + 266 days − the embryo’s age: 263 days for a day-3 embryo, 261 for day 5, 260 for day 6.
  • Ultrasound: due date = scan date + 280 days − the gestational age the scan measured.

The cycle-length step is there because the 280-day rule assumes ovulation on day 14 of a 28-day cycle. Ovulation actually comes about 14 days before the next period, so in a 32-day cycle it happens about four days later, and so does the due date.

Worked examples

  • Last period, 28-day cycle: a period that started on January 1, 2026 gives a due date of October 8, 2026. Full term runs from Oct 1 to October 14, 2026.
  • Last period, 32-day cycle: the same start date gives October 12, 2026, four days later.
  • Conception: conceiving on January 15, 2026 gives October 8, 2026, the same date as the 28-day example, because that’s when its ovulation fell.
  • IVF: a day-5 embryo transferred on February 2, 2026 is due October 21, 2026 (261 days). A day-3 embryo transferred that day would be due October 23, 2026, two days later, because it was fertilized two days after the day-5 embryo.
  • Scan: with that January 1 period, a scan on February 26 (8 weeks by the period) that measures 7 weeks 1 day puts the due date at October 14, 2026, 6 days later. That’s more than the 5 days ACOG allows at that stage, so the scan’s date is used.

Due date by cycle length

For a last period that started on January 1, 2026:

Cycle lengthAdjustmentDue date
24 days−4 daysOctober 4, 2026
26 days−2 daysOctober 6, 2026
28 daysnoneOctober 8, 2026
30 days+2 daysOctober 10, 2026
32 days+4 daysOctober 12, 2026
35 days+7 daysOctober 15, 2026

When an ultrasound changes the date

An early scan is the most accurate way to date a pregnancy: up to 13 weeks 6 days, measuring the embryo’s length is accurate to about 5 to 7 days. ACOG uses the scan’s date instead of the period date when the two differ by more than these limits, counted by the period date on the day of the scan:

Stage at the scanUse the scan if it differs by more thanScan accuracy
up to 8 6/7 weeks5 days±5–7 days
9 0/7 to 13 6/7 weeks7 days±5–7 days
14 0/7 to 15 6/7 weeks7 days±7–10 days
16 0/7 to 21 6/7 weeks10 days±7–10 days
22 0/7 to 27 6/7 weeks14 days±10–14 days
28 0/7 weeks and later21 days±21–30 days

In the third trimester the date changes only if it’s the first scan of the pregnancy. Once an early scan has set your due date, later scans don’t usually change it. A pregnancy with no scan before 22 weeks is considered suboptimally dated, so if your dates are uncertain, an early scan is worth asking about.

What a due date can’t tell you

A due date is an average, not a prediction for your baby. In a study that measured ovulation with daily hormone tests, healthy pregnancies still varied in length by 37 days, even after leaving out preterm births. Dates from a period carry extra uncertainty: ACOG notes that only about half of women remember the first day of their last period accurately, and ovulation doesn’t always fall when the calendar says. That’s why the calculator shows the term windows: early term starts on Sep 17 in the January example, and full term runs through October 14, 2026.

The trimester dates follow ACOG too: the first trimester runs to 13 weeks 6 days, the second from 14 weeks to 27 weeks 6 days, and the third from 28 weeks. The term labels come from ACOG Committee Opinion 579, which replaced the old idea of “term” as a single five-week block because outcomes differ within it.

Wondering when conception happened? The conception calculator works backward from a due date to the likely window. Planning ahead, the ovulation calculator estimates fertile days. For sleep in the months ahead, there’s the sleep calculator.

Sources

This calculator gives an estimate, not medical advice. Your doctor or midwife will confirm your due date, usually with an early ultrasound, and that’s the date to use. If anything about your pregnancy worries you, call them: they would always rather hear from you.

Frequently asked questions

How is my due date calculated?

By convention, a due date is 280 days (40 weeks) after the first day of your last period. That assumes a 28-day cycle with ovulation about two weeks in, so if your cycles are longer or shorter the date moves by the difference. From a conception date it’s 266 days (38 weeks). A period that started on January 1, 2026 gives October 8, 2026.

How accurate is a due date?

It’s the middle of a window, not a deadline. Even when the day of ovulation is known exactly, healthy pregnancies vary in length by more than five weeks. Full term runs from 39 weeks to 40 weeks 6 days, and a baby born any time from 37 weeks is at term. Your care team plans around that window, not the single day.

Why are pregnancy weeks counted from my last period, not from conception?

Because the first day of a period is a date most people know, while the day of ovulation usually isn’t. The count starts about two weeks before conception, so at “4 weeks pregnant” the embryo is about two weeks old. Doctors, midwives and scans all use the same convention, which keeps everyone’s dates in step.

Can my due date change after an ultrasound?

Yes, usually once, after your first scan. ACOG recommends using the scan’s date when it differs from the period date by more than a set number of days: 5 days up to 8 6/7 weeks, 7 days up to 15 6/7 weeks, then 10, 14 and 21 days later in pregnancy. After that, later scans don’t usually move the date: a baby measuring small or large is tracked as growth, not redated.

How do I work out my due date after IVF?

From the transfer date and the embryo’s age. ACOG gives 263 days after a day-3 embryo transfer and 261 days after a day-5 transfer, both 266 days from fertilization; a day-6 embryo follows the same rule, 260 days. A day-5 transfer on February 2, 2026 gives October 21, 2026. It works the same for fresh and frozen transfers.

What if my cycles aren’t 28 days?

The calculator moves the date by your cycle length minus 28, because ovulation comes about 14 days before the next period whatever your cycle length. With a 32-day cycle, a period on January 1 gives October 12, 2026 instead of Oct 8. If your cycles are irregular, the last-period method is unreliable and an early scan is the better guide.

What do early, full and late term mean?

They are ACOG’s labels for when a baby is born: early term is 37 weeks to 38 weeks 6 days, full term 39 weeks to 40 weeks 6 days, late term 41 weeks to 41 weeks 6 days, and post-term 42 weeks or more. Before 37 weeks is preterm.

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