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Due Date Calculator

Find out how your baby's due date is worked out from the first day of your last period, your conception date or your IVF transfer date, and how accurate it is likely to be.

  • Free, no sign-up
  • Updated 1 Oct 2026

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How is the due date calculated?

A pregnancy lasts about 40 weeks, or 280 days, counted from the first day of your last period, not from conception. Conception usually happens about two weeks after the period starts, so by the time you conceive you are already counted as two weeks pregnant.

Due date from your last period (Naegele's rule)

Due date = first day of last period + 280 days

For example, if your last period started on 1 Mar 2026, your due date is 6 Dec 2026.

Other ways to calculate your due date

If you knowDue date
The first day of your last periodAdd 280 days
Your conception dateAdd 266 days
Your IVF transfer date (day 3 embryo)Add 263 days
Your IVF transfer date (day 5 embryo)Add 261 days

If your cycle is longer or shorter than 28 days, you ovulate later or earlier, so the due date moves too. With a 32-day cycle, a last period on 1 Mar 2026 gives a due date of 10 Dec 2026.

The three trimesters

TrimesterWeeksWhat happens
FirstWeek 1 to 13The baby's organs start to form. Nausea and tiredness are common.
SecondWeek 14 to 27The bump grows and you may feel the first movements.
ThirdWeek 28 to birthThe baby gains weight and gets ready for birth.

How accurate is a due date?

A due date is an estimate. Only a small share of babies are born on the exact day; most arrive between 38 and 42 weeks. A pregnancy is called full term from 37 weeks.

An early ultrasound scan, usually between 8 and 14 weeks, measures the baby and is often more accurate than counting from your last period, especially if your cycles are irregular. Your doctor may change your due date after this scan.

Plan ahead for the birth

  • Book your antenatal check-ups and scans early in the first trimester.
  • Check whether your health insurance covers maternity and newborn care, and whether a waiting period applies.
  • Plan your leave from work and how you will get to the hospital.

Frequently asked questions

1.How do I calculate my due date?

Add 280 days (40 weeks) to the first day of your last period. If you know your conception date, add 266 days instead. If your cycle is longer or shorter than 28 days, move the date by the difference. Our pregnancy calculator works this out for you.

2.Why are pregnancy weeks counted from my last period?

Most people know when their last period started but not the exact day they conceived. Counting from the period gives doctors a standard starting point, which is why you are counted as two weeks pregnant at conception.

3.Can my due date change?

Yes. If an early scan shows the baby is bigger or smaller than expected for your dates, your doctor may move your due date. After that, it is usually not changed again.

4.How many weeks pregnant am I?

Count the full weeks since the first day of your last period. Our pregnancy calculator shows this for you, along with your trimester.

5.What if I do not remember my last period?

Use your conception or IVF transfer date if you know it. Otherwise, an ultrasound scan can date the pregnancy accurately, so speak to your doctor.

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