Pregnancy Due Date Calculator

The short answer

• Add 280 days (40 weeks) to the first day of your last period.
• If your cycle is longer or shorter than 28 days, move the date by the difference.
• From a conception date, add 266 days. From a 5-day IVF transfer, add 261 days; from a 3-day transfer, 263 days.
• Pregnancy normally lasts 37 to 42 weeks, so treat the due date as the middle of a window, not an appointment.

How a due date is worked out

The usual method is called Naegele’s rule, after the German obstetrician who described it in the 19th century. Take the first day of your last menstrual period (often shortened to LMP) and add 280 days. A shortcut gives the same answer: add one year, take away three months and add seven days.

The calculator above does this for you, and also shows how many weeks and days pregnant you are today, which trimester you are in and the dates of your scans.

Why weeks count from your period

It surprises many people that pregnancy is counted from the start of the last period, about two weeks before conception. The reason is practical: most people know when their period started, but few know the day they conceived. Counting from the period gives everyone, including midwives and doctors, the same starting point.

So at "4 weeks pregnant", the embryo is about two weeks old. The 40 weeks of pregnancy include roughly two weeks before conception and 38 weeks after it.

Adjusting for your cycle length

Naegele’s rule assumes a 28-day cycle, with ovulation around day 14. The NHS says ovulation happens around 10 to 16 days before the next period, so with a longer cycle you ovulate later, and with a shorter cycle earlier. The calculator moves the due date by the difference between your cycle and 28 days.

The NHS treats cycles of 21 to 35 days as normal. Measure from the first day of one period to the day before the next. If your cycles vary a lot, use an average, and expect the dating scan to give a more reliable date.

From a conception date

If you know the date you conceived, for example from ovulation tests or a single possible date, add 266 days (38 weeks). Conception on 15 September 2026 gives the same due date as the example above: Tuesday 8 June 2027.

Bear in mind that sperm can survive for up to 7 days, so conception can happen several days after sex. The date of sex is not always the date of conception. If you are trying to conceive, the ovulation calculator shows your likely fertile days.

From an IVF transfer

With IVF the timing is known exactly. An embryo transferred on day 5 after egg collection (a blastocyst) is five days past fertilisation, so the due date is 266 − 5 = 261 days after the transfer. For a day-3 embryo it is 263 days.

Your fertility clinic will give you a due date too; use theirs if it differs, as they know the details of your treatment.

From a dating scan

The NHS offers a dating scan at 11 to 14 weeks. The sonographer measures the baby from head to bottom and gives the pregnancy as weeks and days, for example 12+2. That measurement is usually more accurate than a period date, so the scan date becomes your official due date.

Choose "Dating scan result" in the calculator and enter the scan date and the weeks and days from your report.

Reading weeks and days

Midwives write the stage of pregnancy as completed weeks plus days. "12+3" means 12 weeks and 3 days. You are "in your 13th week" at 12+3, because 12 weeks are complete. This is why apps and leaflets can seem a week apart: some count completed weeks, others the week you are in. The calculator uses completed weeks, like your notes.

The three trimesters

Pregnancy is split into three trimesters. The NHS uses these week ranges:

The first trimester is when the baby’s organs form and when many people feel most tired and sick. The second often feels easier, and movements are usually felt in it. The third is about growth and getting ready for birth.

Key dates in pregnancy

For a last period on 1 September 2026, the calculator gives these dates:

1. 29 September 2026 (4+0)Missed period

   Most home tests can detect pregnancy from around now.

2. 17 November to 7 December 2026Dating scan window (11 to 14 weeks)
3. 5 to 25 January 202720-week scan window (18 to 21 weeks)
4. 16 February 2027 (24+0)Viability
5. 18 May 2027 (37+0)Full term
6. 8 June 2027 (40+0)Due date
7. 15 June 2027 (41+0)Induction usually offered
8. 22 June 2027 (42+0)End of the normal range

Your NHS scans

In England, the NHS offers at least two scans. The dating scan, at 11 to 14 weeks, checks how far along you are, whether there is more than one baby, and can include screening for Down’s, Edwards’ and Patau’s syndromes if you choose. The mid-pregnancy screening scan, between 18 and 21 weeks and often called the 20-week scan, looks at how the baby is developing.

The hospital books these, but knowing the windows helps you plan time off. You have the right to paid time off work for antenatal appointments.

24 weeks and viability

The NHS describes 24 weeks as the point at which a baby is considered viable: born now, with the right neonatal care, the baby could survive. Chances improve with every extra week in the womb. It is a milestone many parents feel relieved to reach, but care is always tailored to the individual baby.

Full term, 40 weeks and 42 weeks

• Before 37 weeks: premature (preterm) birth.
• 37 to 42 weeks: the normal range. A baby born in this window is called term.
• 40 weeks: the due date.
• 41 weeks: the NHS offers induction if labour has not started.
• 42 weeks: if you choose not to be induced, you should be offered extra monitoring of the baby.

How accurate is a due date?

Only a small share of babies are born on their due date. Most arrive in the weeks either side, which is why the NHS describes a normal pregnancy as lasting 37 to 42 weeks. First babies often come a little later than later babies, and cycle length, the exact day of ovulation and the baby’s own timing all play a part.

Irregular periods and other cases

A period-based date is less reliable if:

• your periods are irregular, or you do not remember the date;
• you conceived soon after stopping hormonal contraception, or while breastfeeding;
• you had bleeding in early pregnancy that looked like a period.

In these cases, use the calculator for a rough idea and rely on the dating scan for your official date. Tell your midwife if you are not sure of your dates; they may arrange an earlier scan.

Twins and more

The due date is worked out the same way for twins, but twins are more likely to be born early, and the NHS usually plans birth before 40 weeks. Your care team will give you a plan based on the type of twin pregnancy. The key dates in this calculator are for a single baby.

Dates that matter at work (UK)

In the UK, employees must tell their employer about the pregnancy, the due date and when they want leave to start by the end of the 15th week before the due date: around 25 weeks. Maternity leave can start up to 11 weeks before the due date, around 29 weeks.

Your due date also decides your qualifying week for Statutory Maternity Pay. The maternity pay calculator works out what you will get, and the paternity pay calculator does the same for partners. Turn on "Show UK dates" in the calculator to add these dates to your list.

Money and help to plan

A due date is also a budgeting date. The cost of a baby calculator adds up first-year costs and the help you can get. On a low income in the UK, you may be able to claim the Sure Start Maternity Grant from 11 weeks before the due date, and Healthy Start from 10 weeks of pregnancy.

First steps when you find out

• Contact a GP or midwife to book your first (booking) appointment. In England you can often refer yourself to maternity services online.
• Take a daily folic acid supplement until 12 weeks, as the NHS recommends.
• Stop smoking and drinking alcohol; ask your pharmacist before taking any medicine.
• Write down the first day of your last period: you will be asked for it.

When to get help

Key numbers