Pregnancy Due Date Calculator

About the Pregnancy Due Date Calculator

A due date is the first number most people want once a pregnancy is confirmed, and it shapes everything after it: scan appointments, screening tests, leave from work, and the long countdown in between.

It is worth knowing from the start what kind of number it is. A due date is a midpoint, not a prediction of the day. Only about 1 in 25 babies arrives on it. Most are born within a week or two either side, and a birth anywhere from 37 weeks to the end of week 41 counts as term.

This pregnancy due date calculator works out the date from any of three starting points — your last period, a known conception date, or an IVF transfer — and then lays out the window around it and the key dates along the way, so the due date sits in its proper context.

How to Use the Pregnancy Due Date Calculator

Choose what you know.

First day of your last period is the most common starting point. Add your usual cycle length, measured from the first day of one period to the first day of the next. If your cycles vary, use a typical figure.

Conception date works if you know when you conceived, for example from ovulation tracking.

IVF transfer date gives the most precise result. Choose whether the embryo was transferred on day 5 (a blastocyst) or day 3.

How the Due Date Is Worked Out

  From last period:   due date = LMP + 280 days + (cycle length − 28)
  From conception:    due date = conception + 266 days
  From IVF transfer:  due date = transfer + 261 days  (day-5 embryo)
                      due date = transfer + 263 days  (day-3 embryo)

The first line is Naegele's rule, named after a 19th-century German obstetrician. It assumes ovulation on day 14 of a 28-day cycle. A longer cycle means later ovulation, so the due date moves later by the same number of days; a shorter cycle moves it earlier.

All three methods are brought to the same clock — gestational age counted from the last period — which is why they agree when the dates are consistent.

Step-by-Step Example

Last period started 1 March 2026. 28-day cycle.

  1 March 2026 + 280 days  =  6 December 2026

Conception most likely happened around 15 March. An IVF day-5 transfer on 20 March gives the same answer: 20 March + 261 days is also 6 December.

Term runs from 15 November (37 weeks) to 19 December (41 weeks 6 days).

Why Pregnancy Starts Before Conception

This convention surprises almost everyone the first time.

Gestational age is counted from the first day of the last period, about two weeks before ovulation and conception. The reason is historical: before ultrasound, the start of the last period was the one date a woman could reliably report, while the day of conception was usually unknown.

The practical consequence is that the first two "weeks of pregnancy" happen before there is any pregnancy at all. At four weeks pregnant — often around the time of a positive test — the embryo is about two weeks old. Every scan report and appointment letter uses this gestational count, so the calculator does too.

Why a Scan Can Change the Date

A period-based due date assumes a textbook cycle and an accurately remembered date, and neither is guaranteed. Ovulation can come earlier or later than expected even in regular cycles.

A first-trimester ultrasound measures the embryo directly, from crown to rump, and is accurate to within about five to seven days. So clinicians generally move the due date to the scan's when the two differ by more than seven days before 14 weeks. Later scans are less precise for dating, because babies' sizes spread out as pregnancy goes on.

If your clinic has given you a scan-based due date, that is the one to use.

Term, Early and Late

These categories, from the American College of Obstetricians and Gynecologists, replaced a single "term" label because outcomes differ meaningfully across that five-week span. Most maternity units increase monitoring from 41 weeks and offer induction before 42.

Understanding Your Result

Due date is the estimated date, 40 weeks from the start of the dating clock.

Today shows how many weeks and days pregnant you are now, and the trimester.

The window around it gives the term range, from 37 weeks to 41 weeks 6 days.

Conception estimates when conception most likely occurred.

Key dates lists milestones: the end of the first trimester, the halfway point, the start of the third trimester, and the term boundaries.

Worth knowing explains how your cycle length, IVF dating or a scan affects the result.

When Should You Use This Calculator?

After a positive test, to get a first estimate before your booking appointment.

To plan ahead — leave, travel, or when to tell people.

To check a date given to you, or understand why a scan moved it.

Not in place of your clinic's date, which may be based on a scan.

Common Mistakes

Treating the due date as the day. Only about 1 in 25 babies is born on it.

Counting from conception. Pregnancy is counted from the last period, two weeks earlier.

Ignoring cycle length. A 35-day cycle moves the due date a week later.

Using the last day of the period. It is the first day that counts.

Keeping the period date after a scan. An early scan is usually more accurate.

Every pregnancy is different, and your midwife or doctor is the right person for questions about your own dates and care. Nothing here is medical advice.