Pregnancy due date calculator
Estimate your due date from the first day of your last period, a known conception date or an IVF transfer date. The calculator also shows how many weeks pregnant you are today and when the main milestones of your pregnancy fall.
Uses a standard published formula, explained below. The result is an estimate, not medical advice, and its limits are explained further down the page.
Your dates
Standard dating assumes 28 days. Longer or shorter cycles shift the estimate.
Estimated due date
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Currently
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Trimester
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Days remaining
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Estimated conception
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| Milestone | Gestational week | Date |
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Show the working
—Follow your midwife or doctor, not this calculator. A dating ultrasound is considerably more accurate than any date-based estimate, particularly in the first trimester, and your care team's date supersedes this one. Only about four percent of babies arrive on their estimated due date. This tool cannot assess anything about your pregnancy and is not a substitute for antenatal care.
Why pregnancy is counted from your last period
This confuses almost everyone at first. Pregnancy is counted from the first day of your last menstrual period (LMP), which is usually about two weeks before conception. By this convention, you are already counted as two weeks pregnant at the moment of conception.
The reason is practical. Most people know the date of their last period, but very few know the exact date of conception, so doctors standardized on the date that can usually be known.
This is why a 40-week pregnancy involves only about 38 weeks of actual development, and why the conception date shown in the results is about two weeks after your last period.
How the due date is calculated
The standard method, called Naegele's rule, adds 280 days to the first day of your last period. It dates from the early nineteenth century and assumes a regular 28-day cycle, with ovulation on day 14.
That assumption does not fit everyone. Cycles commonly last anywhere from 21 to 35 days, and ovulation timing varies too. The calculator adjusts for your average cycle length by adding or subtracting the difference from 28 days, so a 32-day cycle moves the due date four days later. If your cycles are irregular, this adjustment is only a guess, and an early ultrasound will be far more reliable.
IVF dating is the exception. Because the transfer date is known exactly, an IVF due date is much more accurate than one calculated from a last period.
How reliable is a due date?
Less than an exact date suggests. Only about 4% of babies are born on their due date. A full-term pregnancy is anywhere from 37 to 42 weeks, and most babies are born somewhere in that five-week window.
First pregnancies tend to last slightly longer than later ones. It is best to think of your due date as the middle of a range, and to be prepared for the birth to come a couple of weeks either side of it.
An early ultrasound, especially between weeks 8 and 13, can date a pregnancy to within a few days. If it differs from your LMP date by more than about a week, your care team will usually go by the ultrasound.
The milestone dates
The table shows when key points in your pregnancy fall. The end of the first trimester, at 13 weeks, is when the risk of miscarriage drops considerably. The anatomy scan, usually offered between 18 and 22 weeks, checks the baby's development in detail. Thirty-seven weeks is when a pregnancy counts as full term, and 40 weeks is the estimated due date itself.
These are general reference points. Antenatal care schedules differ between countries and between individual pregnancies, so your doctor or midwife will tell you which appointments apply to you.
Frequently asked questions
How do I calculate my due date from my last period?
Add 280 days, or 40 weeks, to the first day of your last menstrual period. If your cycle is longer or shorter than 28 days, adjust by the difference. The calculator does both for you.
How many weeks pregnant am I?
Count the weeks from the first day of your last period to today. The calculator shows this as your current week and trimester as soon as you enter the date.
How accurate is a due date?
Only about 4% of babies are born on their due date. A full-term pregnancy lasts anywhere from 37 to 42 weeks, so treat the date as the middle of a range.
Why is pregnancy counted from my last period?
Because most people know that date, while the date of conception is usually unknown. It means pregnancy is counted as about two weeks longer than the baby's actual age.
What if my periods are irregular?
Then the estimate is less reliable, because the calculation assumes you ovulated at a predictable time. An early dating ultrasound is much more accurate, and it is the date your care team will use.
What counts as full term?
Thirty-seven weeks and beyond is generally considered full term, although some guidance calls 37 to 38 weeks early term and 39 to 40 weeks full term. A baby born before 37 weeks is preterm.
Should I trust this date over my ultrasound date?
No. An ultrasound in the first trimester can date a pregnancy to within a few days, and it takes priority over any calculation from dates. Always follow the due date your care team gives you.