Why a 'due date' exists at all
A due date is not a birth appointment; it is an anchor. Obstetricians use it to time screening tests, track fetal growth, schedule induction decisions and set expectations for the family. Because the actual duration of pregnancy varies naturally — from about 37 to 42 weeks in healthy pregnancies — the due date is best understood as the center of a window, not the target of a countdown to the minute.
The standard reference point is 280 days, or 40 weeks, measured from the first day of the last menstrual period. This number has a long clinical history, and it remains the foundation of every due date you will see on a pregnancy app or a hospital letter.
That 280-day convention is also why our Due Date Calculator offers three input methods: they all converge on the same 40-week framework but start from whatever information you actually have.
Method 1: LMP (Naegele's rule)
If you know the first day of your last menstrual period, the calculation is famously simple: add 280 days. In practice, Naegele's rule adds one year, subtracts three months, and adds seven days — the same arithmetic wearing a party costume.
The method carries an implicit assumption: a regular 28-day cycle with ovulation on day 14. That assumption holds for many women and fails for many others. Women with longer cycles ovulate later, so the true due date tends to run later than Naegele predicts; women with shorter or irregular cycles see the opposite or simply more scatter.
Accuracy-wise, LMP dating is excellent when the cycle is regular and the date is well remembered — within a few days of the ultrasound estimate in most cases. It degrades quickly when cycles are irregular or the date is fuzzy, which is exactly why clinics do not rely on it alone.
Method 2: known conception date
Some families know the conception date precisely — a charted ovulation, a single act of intercourse that matches the window, or a timed insemination. In that case the math is even more direct: count 266 days, or 38 weeks, forward from conception.
Why 266 and not 280? Because conception marks the start of the embryo's life, and the 14 days before it belong to the follicular phase of the cycle, not to the pregnancy itself. So the 280-day convention = 14 days of 'before' plus 266 days of actual embryonic and fetal development.
This method's accuracy mirrors the precision of its input. If the conception date is truly known — rare outside fertility treatment — it is among the most accurate estimates available. If it is inferred, it inherits the inference's uncertainty.
Method 3: IVF transfer date
In vitro fertilization is the one scenario where the 'age' of the embryo is known to the day, which makes the due date calculation unusually reliable. The rule: take the standard 266 days and subtract the embryo's age at transfer.
For a Day-3 embryo transfer, subtract 18 days, giving a due date of transfer + 266 − 18 = transfer + 248 days. For a Day-5 blastocyst transfer, subtract 19 days, giving transfer + 247 days. The one-day difference reflects how much older the blastocyst is at the moment it enters the uterus.
Because the starting point is a laboratory fact rather than a recalled period, IVF due dates are the most accurate of the three methods — usually matching the ultrasound within two to three days. Our tool labels this clearly so users understand why the method matters.
Why only 4% of babies arrive on their due date
Here is the statistic that surprises everyone: only about 4% of babies are born on their exact due date. About 70–80% are born within two weeks on either side, and the distribution is comfortably wide. Being 'overdue' by a week is medically routine and usually not a cause for alarm — your care team monitors and acts when the risks start to climb.
Ultrasound dating changes the picture meaningfully. A first-trimester scan measuring the crown-rump length is more accurate than LMP arithmetic alone, which is why most clinics finalize the official due date from the scan when the two differ by more than a few days.
What should you do with your calculated date? Mark it on the calendar, plan maternity leave around the middle of the window, and treat the last few weeks as 'any time from now.' Our Pregnancy Week Tracker takes the same due date and turns it into weekly milestones, so the anchor keeps working long after the initial calculation.
Practical tips for using our calculator
Run the tool with whichever input you trust most. If you have both an LMP date and a scan date, use the scan date as the reference and the LMP method as the cross-check. If you are planning and the date is in the future, the tool still works — the trimester and week readouts simply compute from today's date against your anchor.
One habit that saves worry: update your due date whenever a scan refines it, and re-run the calculator with the new anchor. Small changes of a few days are normal and do not change the birth plan.
Finally, remember the disclaimer on every page of BabyPredictor: these tools are for education and planning. Your midwife and obstetrician are the people to consult for the official timeline, and they will always prefer the ultrasound over any app — including ours.
