Due Date Calculator

How to Use the Pregnancy Due Date Calculator

Pick how you want to work out your estimated due date: from the first day of your last period, from your conception date, from an IVF embryo transfer, or from the gestational age on an ultrasound report. The due date calculator shows your due date, how many weeks and days pregnant you are today, your trimester, how many days are left, and the calendar dates of the usual antenatal milestones.

Whether you think of it as a due date, a due calculator or a due days calculator, the arithmetic is the same: everything is counted from a starting date to 40 weeks (280 days), and the calculator does the counting for you.

Due Date Formulas

Worked Example: Last Period on 2 March 2026

Suppose the first day of your last period was 2 March 2026, your cycle is 28 days, and today is 2 October 2026. Adding 280 days gives an estimated due date of Monday 7 December 2026, and conception is estimated at around 16 March 2026. Today you would be 30 weeks 4 days pregnant, in the third trimester, with 66 days to go and about 76.4% of the way through. The two-week window either side runs from 23 November to 21 December 2026.

For the same pregnancy, the typical screening window (11 weeks to 13 weeks 6 days) falls between 18 May and 7 June 2026, the morphology scan window (18 to 20 weeks) between 6 and 26 July 2026, and the glucose tolerance test window (24 to 28 weeks) between 17 August and 13 September 2026. Full term (37 weeks) begins on 16 November 2026. If the cycle were 32 days instead of 28, the due date would move four days later, to Friday 11 December 2026.

Your Due Date Is an Estimate, Not a Deadline

Only a minority of babies arrive on the exact due date. Most arrive within about two weeks either side, which is why term is generally taken to start at 37 weeks and why care teams talk about a window rather than a single day. Your midwife, GP or obstetrician may also adjust your due date after an early dating scan, so if your care provider gives you a different date from this calculator, follow theirs.

Typical Antenatal Timings in Australia

The milestones table shows typical timings in Australian antenatal care: first-trimester screening at about 11 to 13 weeks 6 days, a morphology scan at about 18 to 20 weeks, a glucose tolerance test at about 24 to 28 weeks, and a routine check around 28 weeks. Hospitals, clinics and individual providers differ, and you may have extra visits or scans if you have particular health needs, so treat the dates as a planning aid and confirm each appointment with your care provider.

Frequently Asked Questions

How is my estimated pregnancy due date calculated?

The most common method adds 280 days (40 weeks) to the first day of your last period, which assumes a 28-day cycle with ovulation around day 14. If your cycle is longer or shorter than 28 days, the date is shifted by the difference. A due date is always an estimate of when you will reach 40 weeks, not a prediction of the day your baby will be born.

How do I use a due date calculator from conception?

Choose the conception tab and enter the date you conceived, if you know it. The calculator adds 266 days (38 weeks), because conception happens roughly two weeks after the start of the last period. This works best when conception is well pinned down, for example after tracking ovulation or a single episode of intercourse; otherwise a due date based on your last period or an early ultrasound is generally more reliable.

What if my cycle isn't 28 days? How do I use this due calculator?

Enter your average cycle length, counted from the first day of one period to the first day of the next. The calculator allows 20 to 45 days. If your cycles are very irregular, or you don't remember your last period, an early dating ultrasound is usually the better guide, and your midwife or doctor can advise which date to use.

Is a due date calculator for Australia any different?

The dating rules are the same worldwide, so the calculation doesn't change. What can differ is the timing of antenatal care: this page shows typical Australian timings for the first-trimester screening window, the morphology scan, the glucose tolerance test and the 28-week check, but they vary between hospitals, clinics and providers. Your GP, midwife or obstetrician will confirm the schedule that applies to you.

What do 'weeks pregnant' and 'days to go' mean on a due days calculator?

Weeks pregnant is your gestational age, counted from the first day of your last period (or the equivalent date for your method), so it includes about two weeks before conception. It is shown as whole weeks plus days, for example 30 weeks 4 days. Days to go is the number of days from today until your estimated due date, and the progress bar shows how far through the 40 weeks you are.

How accurate is the due date, and will my baby arrive on it?

Treat it as a guide. Most babies arrive within about two weeks either side of the due date, and many are born a little earlier or later. That is why the calculator shows a window of two weeks before to two weeks after, and why a pregnancy is generally considered term from 37 weeks.

How is the due date worked out after IVF?

With IVF the date of fertilisation is known, so the due date is counted from the embryo transfer. For a day-5 embryo the calculator adds 261 days to the transfer date, and for a day-3 embryo it adds 263 days. Your fertility clinic will usually give you a due date as well, and it is a good idea to use theirs if it differs.

Why might my ultrasound give a different due date?

A scan estimates gestational age from the baby's measurements, and in early pregnancy those measurements are very consistent between babies. If the scan date differs noticeably from the date based on your last period, your midwife or doctor may recommend using the scan date. Enter the gestational age written on your scan report and the scan date in the Ultrasound tab to see what that gives.

When are the first-trimester screening, morphology scan and glucose test?

As typical timings, first-trimester screening is offered at about 11 weeks to 13 weeks 6 days, the morphology (anatomy) scan at about 18 to 20 weeks, and the glucose tolerance test at about 24 to 28 weeks. The milestones table above converts these into calendar dates for your pregnancy. They are guides only, so follow the timing your midwife, GP or obstetrician gives you.