Due Date Calculator

When is the baby due?

Choose how you want to work it out — from your last period, from a known conception date, or from an IVF transfer — enter the date, and the tool gives you the estimated due date, how far along you are today, and the dates along the way.

It also shows the two-week window either side of the due date, because that is where most babies actually arrive.

Everything runs in your browser. Nothing is uploaded, stored or logged. Nothing here is medical advice, and a dating scan is more accurate than any of this.

Naegele’s rule

The standard method, and the one behind almost every due date given anywhere: 280 days — 40 weeks — from the first day of your last menstrual period.

It is named for Franz Naegele, who popularized it around 1812, and it has survived two centuries largely unchanged.

You may have seen the shortcut: take the first day of your last period, add a year, subtract three months, add seven days. That is an approximation of 280 days rather than an equivalent — because months differ in length, it lands a day or two off depending on which months the pregnancy spans. This tool counts actual days.

Naegele’s rule assumes a 28-day cycle with ovulation on day 14. Cycles vary, so the tool lets you adjust: a longer cycle means later ovulation and a later due date, day for day.

Why the count starts before you were pregnant

This confuses almost everyone, and it is worth stating plainly.

Gestational age is measured from the first day of your last period — which is roughly two weeks before conception. So at the moment of conception you are already counted as two weeks pregnant, and by the time a test comes back positive you are typically around four weeks.

It is a convention of convenience. The date of your last period is usually known; the date of conception usually is not. Everything in maternity care — scan timing, screening windows, the 40-week count — runs on this scale, so it is the one worth thinking in.

If you know your conception date, the tool handles it: 266 days rather than 280, the same pregnancy measured from a different starting line.

IVF dates are the reliable ones

With IVF, the one thing normally unknown becomes exactly known: fertilization happened in a lab, on a recorded date.

Day-5 (blastocyst) transfer: 261 days. The embryo is already five days past fertilization, so 266 − 5.
Day-3 transfer: 263 days. Same arithmetic, three days along.

Because there is no guesswork about ovulation, IVF due dates are the most accurate of any method here — more accurate than a last-period date, and typically confirmed rather than corrected by the dating scan.

The due date is a midpoint, not an appointment

This is the single most useful thing to understand about the number.

Fewer than 5% of babies are born on their estimated due date. The rest are spread either side, most within about a fortnight. The date is the middle of a distribution — arriving on it is a coincidence rather than a schedule being kept.

Two implications worth carrying:

Full term is 37 weeks, not 40. From 37 weeks a baby is no longer considered early. That is three weeks before the date everyone fixates on, and it is much less widely known than it should be.

Going past the date is ordinary, particularly for a first pregnancy. Most pregnancies that continue past the due date do so by days, not weeks, and care after 41 weeks is a normal part of maternity practice rather than a sign something is wrong.

Telling people a range rather than a date takes some pressure off the last fortnight — which is a real, if unmeasurable, benefit.

The trimesters

First: weeks 0–13. From the last period through the end of week 13. The pregnancy establishes itself and the major structures form. Early symptoms are usually at their strongest.

Second: weeks 14–27. Often the most comfortable stretch. The anomaly scan falls around 18–21 weeks, and movement usually becomes noticeable somewhere in this period.

Third: week 28 onward. Growth and preparation, with more frequent appointments toward the end.

The exact boundaries vary by a few days between sources — there is no single official definition — so treat them as approximate. What is not approximate is 37 weeks for full term, which is a defined clinical threshold.

Why a scan beats this page

Everything on this page is calendar arithmetic applied to an assumption about when you ovulated.

An ultrasound in the first trimester measures the pregnancy directly, and is accurate to within about five days at that stage. It does not care whether your cycle was 28 days or 34, or whether you ovulated when the textbook says.

So if your dating scan gives a date that differs from the one here, the scan is right and this page is not. Clinicians will typically revise the due date when the two differ by more than about a week — and once revised, that date is the one everything else is scheduled from.

Use this tool for the question “roughly when, and how far along am I now”. Use your clinician’s date for anything that matters.

What this tool does not do

It handles one baby on a standard timeline. It does not adjust for twins or higher multiples, which are typically delivered earlier and dated differently; it does not account for any medical condition affecting timing; and it cannot revise itself from a scan.

It also cannot tell you anything about how the pregnancy is going. It is a calendar, not a measurement. Any concern about your pregnancy is a conversation with a midwife or doctor, not with a calculator.

Frequently asked questions

How is a due date calculated?

By Naegele's rule: 280 days — 40 weeks — from the first day of your last period. It has been the standard since about 1812. It assumes ovulation on day 14 of a 28-day cycle, which is why the tool lets you adjust for a longer or shorter cycle.

Why is it counted from my last period, when I was not pregnant yet?

Convention, and a practical one: the date of your last period is usually known and the date of conception usually is not. It does mean gestational age runs about two weeks ahead of the actual age of the pregnancy — you are counted as "4 weeks pregnant" about two weeks after conception.

How likely is the baby to arrive on the due date?

Under 5%. It is the midpoint of a distribution, not an appointment. Most babies arrive within about two weeks either side, which is why the tool shows that window rather than just the date.

What does full term mean?

37 weeks — three weeks before the due date. From that point a baby is no longer considered early. The distinction matters more than the due date itself, and is much less well known.

How do I calculate a due date for IVF?

From the transfer date, which is the one case where the timing is known exactly. A day-5 blastocyst transfer adds 261 days; a day-3 transfer adds 263. Because there is no guesswork about when fertilization happened, IVF due dates are the most reliable of any method here.

My scan gave a different date. Which is right?

The scan. Dating from a first-trimester ultrasound is more accurate than any calendar arithmetic, because it measures the pregnancy directly rather than inferring it from a cycle that may not have behaved as assumed. If the two disagree, your clinician's date is the one to use.

Is my information stored?

No. Everything is calculated in your browser, and nothing is uploaded, stored or logged.