Pregnancy Due Date Calculator

How a due date is worked out, in plain words

Imagine Maya, who has just seen two lines on a test. Her first question is almost always the same: when is the baby due? She can't say the exact day she conceived, but she remembers the day her last period began. That is enough.

Doctors and midwives count a pregnancy from the first day of the last period. They then add 40 weeks, which is 280 days. The result is the estimated due date. It is a way of putting a date on a calendar, not a prediction of the day a baby will arrive.

That is why you will hear that you are "4 weeks pregnant" when conception happened only two weeks ago. Nobody has miscounted. The clock simply starts earlier, at a day almost everyone can remember.

Naegele's rule and the 280-day idea

The method is named after Franz Naegele, a German obstetrician who wrote about it in the early 1800s. The rule, as it is usually taught, goes like this: take the first day of the last period, count back three months, then add seven days. It works because 40 weeks is a little over nine calendar months.

The shortcut can be a day or two off, because months are different lengths. This calculator doesn't use the shortcut. It counts the days exactly, 280 of them, which gives the same answer every time and handles leap years properly.

Why count from the last period at all, instead of from conception? Because most people don't know the day they conceived, and even people who track carefully can be a few days out. The first day of a period is a clear, observable event. It gives clinics one shared starting point, which makes it easier to compare notes between visits.

A worked example

Let's follow Sofia. Her usual cycle is 32 days, and her last period started on Thursday, 12 March 2026. Here is how the calculator thinks.

With the old shortcut you would count back three months to 12 December, add seven days to reach 19 December, then add her four extra days to get 23 December. That is two days off the exact count, which is fine for planning. Her baby will not read the calendar.

When your cycle isn't 28 days

Ovulation usually happens about two weeks before the next period starts, not on a fixed day 14. So someone with a 35-day cycle ovulates later than someone with a 28-day cycle, and someone with a 24-day cycle ovulates earlier. The due date has to move with them.

The calculator does this by adding or subtracting the difference from 28 days. A 35-day cycle moves the date seven days later. A 24-day cycle moves it four days earlier. It accepts cycles from 21 to 45 days, and you should enter your usual average, not a single odd month.

What about Ruwani, who has irregular cycles that can be 30 days one month and 50 the next? A last-period date is a weak starting point for her. The calculator will give a number, but it is shaky. An early ultrasound, which measures the baby directly, is usually a more reliable way to date her pregnancy, and her doctor or midwife will arrange it.

Due date by conception date, IVF and ultrasound: what's different

Conception date. If you know the day, we add 266 days, which is 38 weeks. This is the same pregnancy counted from a later starting line, and it ends on the same day as the last-period method for a 28-day cycle.

IVF and embryo transfer. Here the date of fertilisation is known quite precisely, but the embryo is already a few days old at transfer. A 3-day embryo is counted as conceived 3 days before the transfer, and a 5-day embryo 5 days before. So the due date is the transfer date plus 263 days for a 3-day embryo, or plus 261 days for a 5-day embryo. Clinics have their own dating rules and their own records, so if your clinic has given you a date, use theirs and treat this page as a cross-check.

Ultrasound. Early scans can measure the baby, and those measurements are then converted into a gestational age. If your scan report says 12 weeks and 3 days on a particular day, we work forward to 40 weeks 0 days from there. This is useful when the last period is uncertain.

Already know your date? Enter it and we work backwards, showing the week you are in and the first day of the last period that would match it.

Why a scan can give a different date, and who decides

Priya, a mother of two, was sure of her dates. "I know my body," she said. Her scan, though, put her about nine days behind her own count. This happens more often than people expect.

There are ordinary reasons. Ovulation may have come later than the cycle suggested, or a light bleed may have been mistaken for a period.

In early pregnancy, scan measurements are considered the most reliable way to date a pregnancy, because babies grow at very similar rates in the first three months. Guidelines, such as those from the American College of Obstetricians and Gynecologists, generally suggest keeping the last-period date if it agrees with the scan within about a week, and moving to the scan date if the gap is larger. The exact limits depend on how early the scan was.

Who decides? Your doctor or midwife. They look at your dates, your scan, and your own history, and then settle on one working due date. Once that date has been set, most care teams keep it for the rest of the pregnancy, even if later scans suggest a slightly different size. If your clinic's date differs from this calculator, theirs is the one to follow.

The three trimesters at a glance

Pregnancy is split into three parts to make it easier to talk about. The boundaries are conventions, not medical turning points, and different sources round them a little differently. Here is how this calculator counts them.

Counted on a calendar, 40 weeks is nearer to nine months and a week, which is one reason people's own counts drift from the weeks clinics use.

How accurate is a due date?

The honest answer is that it is an estimate, and a good one for planning, but only a small minority of babies are born on the exact day. It is often quoted as roughly one in twenty. Most arrive in the weeks on either side.

Think of it as a forecast for a wedding "around the end of the month". You'd plan around it, but you wouldn't be shocked if it moved by a few days. A healthy pregnancy can end anywhere from 37 weeks to just past 41, which is why this page shows a window and not only a date.

One more thing, and it matters. For some people, this date arrives with joy. For others it comes with worry, mixed feelings, or a pregnancy that wasn't planned. For some it follows a hard past experience. All of that is allowed. Use the dates in whatever way is useful to you, and take the pace that suits you.

Early term, full term, late term and post-term

Not so long ago, any baby born between 37 and 42 weeks was simply "term". Outcomes differ a little across that range, so the groups are now named more precisely. These are the definitions used by the American College of Obstetricians and Gynecologists and followed by many health services.

A baby born before 37 weeks is called preterm, and the World Health Organization uses the same cut-off. Early term is 37 weeks 0 days to 38 weeks 6 days. Full term is 39 weeks 0 days to 40 weeks 6 days. Late term is 41 weeks 0 days to 41 weeks 6 days. Post-term means 42 weeks 0 days or more.

The labels are for doctors and statisticians. They are not grades. If you are past 40 weeks, your care team will usually talk with you about how you both are and what to do next.

Questions to ask at your antenatal clinic

A first visit can feel like a blur, and many people leave thinking of what they forgot to ask. A note on your phone, or a few lines in the pregnancy record you're given, can help. Here are some practical questions, and you're welcome to ignore any that don't apply.

Which due date are you using for my care, and is it based on my last period or a scan? When are my next visits and tests? Which scans do you usually offer, and roughly when? Whom do I contact if I'm worried between visits? What symptoms should make me call the same day? What should I know about where and how I'll give birth?

In many places, mothers are also supported by a public health midwife, who can be a good first person to ask about routine care in your area. Services differ between places, so the safest approach is to ask your own clinic or midwife how things work for you.

If you are the husband or partner of someone who is pregnant, the most useful thing may be to ask what help she wants, and to come to appointments if she'd like you there.

When to call your doctor or midwife straight away

Most pregnancies have a few odd moments and a lot of ordinary discomfort. Still, some signs should never wait for your next appointment. This list is general, based on guidance from health services such as the NHS and ACOG, and it isn't complete. If you are worried about anything, trust that feeling and make the call.

If you are unsure whether something counts, ring and ask. Nobody who looks after pregnant women will think you're wasting their time.

Frequently asked questions

Medical disclaimer

This calculator and article are for general education. They give an estimate based on standard dating methods and do not diagnose anything, replace a scan, or replace the advice of your doctor or midwife. If your clinic has given you a due date, follow it. If you are worried about your health or your baby's, contact a qualified health professional.