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Due Date Calculator

Estimate your baby's due date from your last period, conception or ultrasound — plus current week and trimester.

✓ Last reviewed: Sources: ACOG Committee Opinion No. 700 (2017) — Methods for Estimating the Due Date (incl. ART: day-3 embryo +263, day-5 +261, fresh IVF +266)

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Naegele's rule assumes a 28-day cycle. If yours is regularly longer or shorter (typical range 21-35 days), we adjust the due date accordingly.

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What the due date calculator tells you

This due date calculator answers the two questions almost everyone asks at the start of a pregnancy: when is my baby due, and how many weeks pregnant am I right now? Enter one date and you get both — an estimated due date (the EDD, or estimated date of delivery) plus your current week and day of pregnancy, your trimester, and how many weeks are left to go.

Most tools online split this in two: one page for the due date, another for “how far along am I”. Here it’s a single calculator, and it works from whichever starting point you actually have:

  • your last menstrual period (LMP) — the standard method,
  • your conception date, if you know it,
  • an ultrasound (dating scan) — the gestational age your sonographer measured,
  • an IVF embryo transfer — day 3, day 5 or day 6.

Everything it shows is an estimate to plan around, not a diagnosis — see the accuracy section and FAQ below.

How to use the due date calculator

  1. Pick your method. Last menstrual period is the default and the one most people use. Switch to conception, ultrasound or IVF if that’s the date you have.
  2. Enter the date. For LMP, use the first day of your last period, not the day it ended. For an ultrasound, add the gestational age the scan reported (for example 12 weeks and 3 days).
  3. Read your results. The estimated due date appears instantly, along with how many weeks pregnant you are today, your trimester, and the days remaining.
  4. Adjust your cycle length (optional). In the advanced options for the LMP method you can set your average cycle length. The default is 28 days; a longer or shorter cycle shifts the due date (see below).

The four ways to calculate a due date

All four methods land on the same 40-week timeline — they just start the clock from a different, known point.

From your last menstrual period (LMP)

This is the classic method. By convention, pregnancy is counted as 280 days (40 weeks) from the first day of your last period — the approach ACOG and the NHS both describe. So:

Due date = first day of last period + 280 days

The 19th-century “Naegele’s rule” (subtract 3 months, add 7 days and a year) is the historical shortcut for the same idea. We use the plain 280-day count instead, because it’s unambiguous — the calendar shortcut can drift by a day around leap years, and 280 days is exactly what the guidance means.

Cycle-length correction. The 280-day count assumes a 28-day cycle with ovulation on day 14. If your cycle is reliably longer or shorter, ovulation — and so conception — shifts, and we move the due date to match:

Due date = last period + 280 days + (your cycle length − 28)

A 35-day cycle pushes the date 7 days later; a 21-day cycle pulls it 7 days earlier. This adjustment (sometimes called Parikh’s correction) is a helpful approximation, not an exact law — the length of the phase after ovulation varies more than the old textbooks assumed — so treat a cycle-corrected date as orientational.

From your conception date

If you know when you conceived (an ovulation date, or a single known encounter), pregnancy runs about 266 days (38 weeks) from conception:

Due date = conception date + 266 days

Why 266 and not 280? Counting from your last period adds roughly the two weeks before ovulation, when you weren’t yet pregnant but the gestational clock is already ticking. 280 − 14 = 266.

From an ultrasound (dating scan)

A dating scan measures the baby directly and reports a gestational age — “12 weeks and 3 days”, for example. From that we work backwards to the due date:

Due date = scan date + (280 days − gestational age at the scan)

An early ultrasound is generally the most reliable way to date a pregnancy, which is why your doctor may set your official due date from the scan rather than from your last period.

From an IVF embryo transfer

With IVF the exact age of the embryo is known, so dating is more precise. We count from the transfer date, subtracting the embryo’s age from the 266-day conception clock:

  • Day 3 transfer (cleavage-stage): due date = transfer date + 263 days
  • Day 5 transfer (blastocyst): due date = transfer date + 261 days
  • Day 6 transfer (blastocyst): due date = transfer date + 260 days

Whether the embryo was fresh or frozen makes no difference to the date — freezing pauses the embryo’s development, so a day-5 blastocyst is a day-5 blastocyst whenever it’s transferred. Only the embryo’s age at transfer changes the maths.

How we count your week of pregnancy and trimester

“How many weeks pregnant am I” uses the same 40-week clock. Whichever method you picked, we anchor it to the standard timeline (your due date minus 280 days) and count forward to today:

  • Weeks pregnant are counted as completed weeks, so 14 weeks and 6 days still reads as “week 14”, not 15 — the same way clinics write it (often as “14+6”).
  • Trimesters follow the common convention: first trimester runs to the end of week 13 (13 weeks 6 days), second from 14 weeks 0 days to 27 weeks 6 days, third from 28 weeks onward. Trimester boundaries aren’t a formal medical standard, so different sources draw them a little differently.
  • Near the end, the calculator also labels the birth window using the ACOG/SMFM categories: early term (37–38 weeks), full term (39–40 weeks), late term (41 weeks) and postterm (42 weeks and beyond).

A worked example, step by step

Say your last period started on 1 January 2026, with a typical 28-day cycle.

  1. Due date = 1 January 2026 + 280 days = 8 October 2026.
  2. Checking in on 15 April 2026: that’s 104 days since 1 January, which is 14 completed weeks and 6 days — so you’re 14 weeks and 6 days pregnant, in your second trimester.
  3. Days left: from 15 April to 8 October is 176 days — about 25 weeks and 1 day until your due date.
  4. Progress: 104 of 280 days = 37.1% of the way through.

Change the inputs and the date moves in step with the method:

  • Longer cycle: last period 1 February 2026 with a 35-day cycle → 1 February + 280 + 7 = 15 November 2026.
  • From conception on 1 May 2026 → 1 May + 266 = 22 January 2027.
  • From an ultrasound on 15 April 2026 reading 12 weeks 3 days → 15 April + (280 − 87) = 25 October 2026.
  • IVF day-5 transfer on 1 May 2026 → 1 May + 261 = 17 January 2027 (a day-3 transfer would give 19 January; a day-6 transfer, 16 January).

How accurate is a due date?

A due date is a single best estimate, and babies rarely read the memo. Only about 5% — roughly 1 in 20 — arrive exactly on their due date. Around 60% are born within a week either side of it, and about 90% within two weeks. A full pregnancy anywhere from 37 to 42 weeks is considered normal.

That’s why the 280-day figure is best understood as an agreed clinical convention rather than a biological certainty, and why your doctor or midwife has the final say — usually setting or confirming the date from an early ultrasound, which measures the baby directly. Treat the result here as a reliable starting point for planning, and let your care team confirm and, if needed, revise it. If you’re pregnant through IVF, carrying multiples, or being followed for any higher-risk reason, the date your clinician gives you is the one to go by.

Frequently asked questions

How is my due date calculated?

By default it’s counted as 280 days (40 weeks) from the first day of your last menstrual period — the standard clinical convention used by ACOG and the NHS. If you calculate from conception instead, it’s 266 days; from an ultrasound, it’s worked backwards from the gestational age measured at the scan; and from an IVF transfer, it’s the transfer date plus 263, 261 or 260 days for a day-3, day-5 or day-6 embryo.

How accurate is a due date calculator?

It’s a good estimate, not a fixed appointment. Only about 5% of babies are born exactly on their due date; roughly 60% arrive within a week of it and about 90% within two weeks. Any birth between 37 and 42 weeks is considered full-length. So use the date to plan, but expect the real day to land somewhere in a two-week window around it.

How many weeks pregnant am I?

Your week of pregnancy is counted from the first day of your last period, in completed weeks — so 13 weeks and 6 days is still “week 13”. This calculator shows it automatically once you enter your date: for a last period on 1 January 2026, on 15 April 2026 you’d be 14 weeks and 6 days pregnant, in the second trimester, about 37% of the way through.

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

The standard due date is counted from the first day of your last period, which is about two weeks before conception. That’s why the “40 weeks” of pregnancy includes roughly two weeks when you weren’t yet pregnant. If you count from conception instead, the pregnancy is 38 weeks (266 days). This calculator supports both — pick “conception date” if that’s the date you know.

How many weeks is a full-term pregnancy?

Full term is 39 to 40 weeks. Since 2013, obstetric guidance (ACOG/SMFM) splits the end of pregnancy into early term (37–38 weeks), full term (39–40 weeks), late term (41 weeks) and postterm (42 weeks and beyond). A birth anywhere from 37 to 42 weeks is within the normal range; before 37 weeks is considered preterm.

Can my due date change during pregnancy?

Yes. Your first estimate comes from your last period, but an early dating ultrasound measures the baby directly and is usually more accurate. If the scan and your last-period estimate differ by more than a set number of days, your doctor will update the official due date to the scan’s. This is routine and doesn’t mean anything is wrong — it’s simply a more precise measurement replacing an estimate.

How does the calculator work for an IVF pregnancy?

IVF dating is more precise because the embryo’s exact age is known. Choose the IVF method and enter your transfer date and the embryo’s age: the due date is the transfer date plus 263 days (day 3), 261 days (day 5) or 260 days (day 6). Whether the embryo was fresh or frozen doesn’t change the result — only its age at transfer does. As always, your clinic confirms the final date.

Does a twin or multiple pregnancy change the due date?

No — the estimated due date is calculated the same way regardless of how many babies you’re carrying. What changes is that twins and multiples statistically arrive earlier, often around 36 to 37 weeks, so the calculated 40-week date is more of a reference point than a target. Your doctor will guide the timeline for a multiple pregnancy.

Sources