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Pregnancy Due Date Calculator

Estimate your due date from your last period, conception date, or a known due date - and see exactly how far along you are.

Due Date Calculator

28 days is the default. A longer cycle shifts the due date later.

Estimated due date

Jun 9, 2027

Wednesday, June 9, 2027
0 weeks 0 daysTrimester 1
Week 0Week 13Week 27Week 40

How far along

0w 0d

Days to go

280

Trimester

1 of 3

Conception

Sep 16, 2026

Estimated

Key dates

First trimester ends
Dec 2, 2026
Second trimester ends
Mar 10, 2027
Estimated due date
Jun 9, 2027

Only about 4% of babies arrive on their exact due date. A first-trimester ultrasound is more accurate than any calculation and takes precedence if the two disagree.

How the due date calculation works

A due date is an estimate of when a pregnancy reaches 40 weeks. The standard method, used by clinicians worldwide, is Naegele's rule:

Due date = first day of your last menstrual period + 280 days

Two hundred and eighty days is forty weeks. The rule has a quirk that confuses almost everyone the first time they meet it: pregnancy is dated from your last period, not from conception. Since ovulation typically occurs about 14 days after the period begins, you are already counted as "two weeks pregnant" at the moment of conception. Actual gestation is closer to 266 days.

A worked example. If your last period began on January 1, 2025, adding 280 days gives a due date of October 8, 2025. Conception would have occurred around January 15, and on June 1 you would be 21 weeks and 4 days pregnant — comfortably into the second trimester.

Cycle length changes the answer. Naegele's rule assumes a 28-day cycle. If yours runs 35 days you ovulate about a week later, so the due date shifts about a week later too. The calculator above adjusts for this, which matters because an inaccurate due date affects how growth is assessed and when interventions are considered.

The trimesters divide the pregnancy as follows:

  • First trimester: weeks 1-13 (through day 91)
  • Second trimester: weeks 14-27 (through day 189)
  • Third trimester: week 28 to birth

The most useful thing to know about due dates is how imprecise they are. Only about 4% of babies are born on their exact due date. Roughly 80% arrive within the two weeks either side, and a first pregnancy averages a few days past 40 weeks. A due date is the centre of a distribution, not an appointment.

If you conceived through IVF, dating is far more precise, because the transfer date is known exactly. For a day-5 blastocyst transfer, the due date is the transfer date plus 261 days. Use the conception-date option above and enter the date of transfer.

Your provider will confirm dating with a first-trimester ultrasound, which is accurate to within about five days and takes precedence over the calculated date if the two disagree by more than a week.

Disclaimer: This calculator provides an estimate for general information only and is not medical advice. Only about 4% of babies arrive on their exact due date. Your healthcare provider will confirm your due date with an ultrasound.

Frequently asked questions

How is a due date calculated?

Naegele's rule adds 280 days - 40 weeks - to the first day of your last menstrual period. It assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the due date shifts accordingly, which this calculator adjusts for.

Why is pregnancy dated from my last period rather than conception?

Because the date of the last period is something you can usually identify, while the exact moment of conception is not. The consequence is that you are considered two weeks pregnant at conception, and the actual gestation period is around 266 days rather than 280.

How accurate is a due date?

Only about 4% of babies arrive on their exact due date, though roughly 80% are born within two weeks either side. A first-trimester ultrasound is the most accurate dating method, to within about five days, and takes precedence if it differs from the calculated date.

How do I calculate a due date after IVF?

IVF dating is precise because the transfer date is known. For a day-5 blastocyst transfer, add 261 days to the transfer date; for a day-3 transfer, add 263 days. Use the conception-date option above and enter your transfer date.

What week of pregnancy am I in?

Count the days since the first day of your last period and divide by seven. The whole number is your completed weeks. Clinicians write this as "21 weeks and 4 days", meaning you have completed 21 weeks and are four days into the 22nd.

When do the trimesters start and end?

The first trimester runs from week 1 through the end of week 13, the second from week 14 through the end of week 27, and the third from week 28 until birth. Some sources shift these boundaries by a week; the split used here is the most common clinical convention.

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