Pregnancy Due Date Calculator
Estimate your due date from your last period, conception, IVF transfer or a dating scan.
Your dates
First day of your last menstrual period
An early dating ultrasound is more accurate than any date-based calculation, and if your clinic has given you a due date from a scan, that is the one to use. This tool reproduces the arithmetic; it does not replace antenatal care.
This estimates a due date from the first day of your last period, or from a conception or IVF transfer date.
It also shows how many weeks pregnant you are and which trimester that falls in.
How a due date is worked out
A due date is estimated by Naegele's rule: add 280 days, which is 40 weeks, to the first day of your last menstrual period.
The part that confuses everyone is that dating starts from the last period, not from conception. Conception happens around two weeks into that count, so on the day you conceive you are already counted as two weeks pregnant. Actual gestation from conception is about 38 weeks, not 40.
It is called an estimated date for good reason. Very few babies arrive exactly on it. Term is defined as a range, 37 to 42 weeks, and the due date is the midpoint of a distribution rather than a deadline.
The first two weeks of a 40-week pregnancy happen before conception. It is a dating convention, chosen because the last period is a date most people can identify.
What to enter
- First day of last period
- The first day of full flow. Day one of the count, and the most common basis for dating.
- Cycle length
- Naegele's rule assumes 28 days. A longer cycle means later ovulation, so the due date shifts later.
- Conception or IVF transfer date
- If you know it. IVF dating is the most precise available, because the timing is known exactly.
- Ultrasound date
- A first-trimester scan is the most accurate method after IVF, and clinicians will often redate from it.
The trimesters and key milestones
- First trimester
- Weeks 1-13. Organ development, and when most early symptoms appear.
- Second trimester
- Weeks 14-27. Often the most comfortable stretch.
- Third trimester
- Weeks 28-40. Rapid growth and preparation for birth.
- Full term
- 39 weeks 0 days to 40 weeks 6 days. Early term is 37-38 weeks and late term is 41.
What this assumes
Naegele's rule assumes a 28-day cycle with ovulation on day 14. Longer or shorter cycles shift the date accordingly.
This is an estimate. A first-trimester ultrasound is more accurate, and clinicians will use it to redate if it differs substantially.
How to calculate your due date
Add 280 days to the first day of your last period, then adjust for cycle length if it is not 28 days.
- 280 days
- 40 weeks, counted from the last period rather than conception
- adjustment
- Add (cycle length − 28) days for a longer cycle
Identify day one. The first day of full flow of your last period, not spotting.
Add 280 days. Or use the shortcut: subtract three months, add seven days, add a year.
Adjust for cycle length. A 32-day cycle means ovulation four days later, so add four days to the estimate.
Expect it to be revised. A first-trimester ultrasound is more accurate, and your clinician may redate from it. That is routine, not a problem.
See a worked example: why you are 'two weeks pregnant' at conception
- Basis
- First day of last period
- Cycle
- 28 days
Due date: last period + 280 days, which is 40 weeks.
Ovulation and conception happen around day 14, so week 2 of that count.
So gestation from conception is about 38 weeks, while pregnancy is described as 40.
On a 32-day cycle, ovulation is day 18 rather than 14, so the due date moves four days later.
280 days from the last period, 38 weeks from conception
Frequently asked questions
- Why is pregnancy dated from my last period?
Because it is a date most people can identify, and conception usually is not. Ovulation and fertilisation are rarely observed directly.
The consequence is that the first two weeks of a 40-week pregnancy happen before conception, and you are counted as two weeks pregnant on the day you conceive.
- How accurate is a due date?
It is the midpoint of a range rather than a prediction. Very few babies are born exactly on their due date, and full term spans 37 to 42 weeks.
A first-trimester ultrasound is the most accurate dating method after IVF, which is why clinicians often revise the date after an early scan.
- What if my cycle isn't 28 days?
Naegele's rule assumes 28 days with ovulation on day 14. A longer cycle means later ovulation and a later due date.
Add the difference: a 32-day cycle shifts the estimate four days later. A 24-day cycle shifts it four days earlier.
- How is an IVF due date calculated?
From the transfer date and the age of the embryo, which makes it the most precise dating available since the timing is known exactly.
For a day-5 blastocyst transfer, the due date is 261 days from transfer. Your clinic will give you the date rather than you needing to work it out.
- When does each trimester start and end?
First: weeks 1-13. Second: weeks 14-27. Third: weeks 28 to birth.
The boundaries are conventions and different sources place them slightly differently. What matters clinically is the week count, not which trimester label it falls under.
- What does 'full term' actually mean?
Since 2013 the terminology has been more specific: early term is 37-38 weeks, full term is 39 weeks 0 days to 40 weeks 6 days, late term is 41 weeks, and post-term is 42 weeks or more.
The change was made because outcomes at 37 weeks differ measurably from those at 39, and calling both simply "term" obscured that.
Problems people actually run into
Treating the due date as a deadline
It is a single point in a distribution spanning several weeks. Most births happen either side of it, and arriving before or after is normal rather than a problem.
The fixation on one date makes the final fortnight harder than it needs to be. Think of it as a due month.
Counting weeks from conception
"Six weeks pregnant" means six weeks from your last period, which is about four weeks from conception. Mixing the two makes early scan results confusing.
Every clinical measurement uses the last-period count. Sticking to that convention keeps your dates consistent with everything you are told.
Results are estimates for general information only and are not professional financial, medical, or legal advice. Read our full disclaimer.
Sources
- Committee Opinion 700: Methods for Estimating the Due Date · American College of Obstetricians and Gynecologists
- Committee Opinion 579: Definition of Term Pregnancy · American College of Obstetricians and Gynecologists
Last updated: September 13, 2026
