Due date calculator
Enter the first day of your last period to find your estimated due date and how many weeks pregnant you are.
Your dates
Your due date: how it is calculated and how accurate it is
A due date is an estimate, not a date. It is calculated in a way that makes it a useful reference for tracking your baby’s development and scheduling appointments. This calculator gives you an approximate due date and week of pregnancy. Your official due date is set by your doctor, usually with the help of an ultrasound.
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How the due date is calculated
The most common method starts from the first day of your last menstrual period and adds 280 days, or forty weeks. That calculation assumes a 28 day cycle with ovulation around day fourteen. It means a due date based on your last period actually measures pregnancy from a date roughly two weeks before conception, which confuses many women the first time they hear which week they are in.
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How accurate it is
Very few babies arrive exactly on the due date, roughly one in twenty. The vast majority of births happen within about two weeks either side of it, and all of that is normal. It is more useful to treat the due date as the middle of the period in which your baby is expected than as a date to have everything ready for.
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Ultrasound versus the period calculation
A first trimester ultrasound measures the size of the fetus and is the most accurate way to estimate gestational age, because in that period all babies grow very similarly. If the date from your last period and the date from an early ultrasound differ by more than a few days, doctors usually take the ultrasound date. Later scans estimate the baby’s size but no longer move the due date, because growth varies more later in pregnancy.
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If your cycles are irregular
The last period calculation assumes a regular cycle, so it easily misses with longer, shorter or unpredictable cycles. The same applies if you conceived soon after stopping hormonal contraception, or if you do not know the exact date of your last period. In those cases an early ultrasound is considerably more reliable and the due date is set from it.
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What early, full and post term mean
Pregnancy is not divided simply into before and after the due date. Birth between 37 and 39 weeks is considered early term, between 39 and 41 weeks full term, and after 42 weeks it is described as post term. The distinction exists because outcomes differ, which is why doctors monitor pregnancy differently as you approach the upper limit.
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What the due date means for training
The week of pregnancy is useful for planning training, because adjustments do not arrive on a date but gradually. Prolonged lying on your back is dropped by around sixteen weeks. Through the second trimester abdominal exercises change and the emphasis moves to stability. In the third, sessions get shorter, intensity comes down and positions become more stable. The due date is a useful framework, but how you feel that week always takes priority.
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How a kinesiologist helps
A kinesiologist plans by week of pregnancy rather than handing you one generic plan for all nine months. Exercises change before they become uncomfortable, the pelvic floor and abdominal wall are tracked throughout, and the last trimester is used to prepare for birth and for an easier recovery afterwards.
Written by Ozana Brkić, MSc in Kinesiology, a certified kinesiologist specialising in prenatal and postnatal training. Content last reviewed: August 2026.
Common questions about due dates
How accurate is a calculated due date?
Roughly one baby in twenty arrives exactly on the due date. The vast majority of births happen within two weeks either side of it. The due date is most useful as the middle of the expected window rather than as a date.
Why is the due date calculated from my last period rather than conception?
Because the date of your last period is almost always known and the exact date of conception rarely is. As a result pregnancy is measured from a date roughly two weeks before conception, which is why you are already formally in week five or six when a test turns positive.
What if my cycles are irregular?
Then the last period calculation is unreliable. An early first trimester ultrasound gives a considerably more accurate estimate, and your doctor will set the due date from it.
Can an ultrasound change my due date?
Yes, if the difference is more than a few days and the scan is from the first trimester. Later scans estimate the baby’s size, but the due date is generally not changed from them.
What does full term mean?
Full term is the period between 39 and 41 weeks. Birth between 37 and 39 weeks is called early term, and after 42 weeks pregnancy is described as post term.
How does my due date affect training?
Through the week of pregnancy, because adjustments are introduced gradually: dropping prolonged lying on your back around sixteen weeks, changing abdominal exercises through the second trimester, and shorter, easier sessions in the third. How you feel that week still matters more than the calendar.
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