Your due date and how far along you are, from your last period.
Estimated due date
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How far along today
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Trimester
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Days to go
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Conception around
Pregnancy timeline
Stage
Weeks
Dates
An estimate, not a diagnosis. Only about 4% of babies arrive on their due date, and a dating ultrasound in the first trimester is far more accurate than any calendar. Always follow the date your midwife or doctor gives you.
How this works
Naegele's rule
Due date = first day of last period + 280 days
Adjusted = + (cycle length − 28) for longer or shorter cycles
Conception ≈ last period + cycle length − 14
Stage
Weeks
Notes
First trimester
0 – 13
Dating scan usually 8 to 14 weeks
Second trimester
14 – 27
Anomaly scan around 20 weeks
Third trimester
28 – 40
Full term from 37 weeks
Early term
37 – 38
Full term
39 – 40
Post term
42+
Induction usually discussed
Pregnancy is counted from the first day of your last period, not from conception — so you are already counted as two weeks pregnant at conception.
280 days is 40 weeks. Actual gestation from conception is about 38 weeks.
Naegele's rule assumes a 28-day cycle; a longer cycle pushes the due date later, which this adjusts for.
Only about 4% of babies are born on the estimated date; most arrive within two weeks either side.
How is a due date worked out?
Naegele's rule adds 280 days to the first day of your last period. If your cycle is longer or shorter than 28 days, the date shifts by the difference.
Why does pregnancy start before conception?
Dating runs from the first day of your last period because that date is known, while conception usually is not. It means you are counted as about two weeks pregnant on the day you conceive.
How accurate is the estimate?
Roughly 4% of babies arrive on the exact date, and most within two weeks either side. A first-trimester dating scan is more accurate and will usually override a calendar estimate.
How do I find my last period date if I already know my due date?
Subtract 280 days (40 weeks) from the due date, then adjust by the difference between your cycle length and 28 days. This calculator works forward from your last period to your due date — run the same formula in reverse to go the other way.
Is this the same as a perinatology calculator?
Yes — "perinatology calculator" is another common name for a pregnancy due date calculator built on Naegele's rule, the same method used here.