A due date is the first day of the last menstrual period plus 280 days, and gestational age is counted from that same day. Because conception happens roughly two weeks later, a pregnancy described as four weeks along contains an embryo about two weeks old. The convention exists because the date of a period is knowable and the date of conception usually is not.
That two-week offset is the source of most of the confusion about pregnancy dating, and it is a measurement convention rather than a biological claim.
Where does the 280 days come from?
Naegele’s rule, in use since the early nineteenth century: take the first day of the last period, add a year, subtract three months and add seven days. It arrives at the same place as adding 280 days and it carries two assumptions worth knowing.
The first is that ovulation happens on day 14, which is only true for a 28-day cycle. On a 35-day cycle ovulation is nearer day 21, so the due date moves about a week later — which is why a dating calculator asks for cycle length rather than assuming it.
The second is that the cycle is regular. Where it is not, an early ultrasound is a considerably better dating method, and it is what clinical practice relies on.
How likely is the date itself?
Not very. Only about 4 per cent of babies arrive on their estimated due date, and term is anywhere from 37 to 42 weeks — so a fortnight either side is entirely ordinary rather than early or late.
| Week | What it marks |
|---|---|
| 24 | The legal threshold of viability in many countries |
| 37 | Full term; birth is no longer considered premature |
| 40 | The estimated due date |
| 42 | The usual upper limit before induction is discussed |
Twenty-four and 37 weeks matter more than 40 does. The first is where neonatal care changes substantially and the second is where the clinical picture changes, whereas the due date is the midpoint of a distribution that nobody expects to hit exactly.
How is the fertile window worked out?
Backwards from the next period rather than forwards from the last one. Ovulation happens roughly one luteal phase before the next period — cycle length minus about 14 days — so on a 28-day cycle that is day 14 and on a 35-day cycle nearer day 21.
The window is the five days before ovulation plus the day itself, and it is asymmetric for a physical reason: sperm survive up to five days in the reproductive tract while an egg is viable for about twenty-four hours. So the days before ovulation matter and the days after it very quickly do not.
That asymmetry is also why waiting for a positive ovulation test to try is late rather than timely. The test detects the hormone surge that precedes ovulation by a day or so, which is near the end of the window rather than the start of it.
What is corrected age, and when does it stop?
Corrected age subtracts the weeks a baby was born early, because development runs from conception rather than from birth. A baby born six weeks early who is eight weeks old has a corrected age of two weeks.
It exists to avoid an unfair comparison. A baby born ten weeks early has had ten fewer weeks to grow, and expecting them to sit, smile or crawl on the chronological schedule sets a misleading benchmark for everyone involved.
The correction is usually applied for the first two years and then dropped, by which point the gap has generally closed. Clinicians differ on exactly when, which is one more reason the figure is a guide rather than a verdict.
Why weeks and then months?
Because change is rapid at the start and slows. Weeks give useful resolution for the first few months, when a fortnight is a substantial fraction of a life; months are adequate afterwards, when it is not.
The switch is a convention rather than a milestone, and it is why a paediatric chart and a nursery form can describe the same baby in different units without either being wrong.
Questions people ask
Why is a due date given as one day? Because a single date is easier to plan against than a range, and because clinical decisions after 41 or 42 weeks need a reference point. It is the middle of a distribution, not a prediction.
Does a scan change the date? Frequently, and where it does the scan date is generally the one used. Early scans date a pregnancy more accurately than a remembered period, particularly with an irregular cycle.
Is a 28-day cycle normal? It is the average and a wide range around it is ordinary. Cycles between roughly 21 and 35 days are common, and it is the length rather than the number 28 that matters for dating.
Can these tools tell me anything about a specific pregnancy? No. They apply a population convention to a set of dates, and anything that matters clinically needs a clinician looking at the actual pregnancy.
The offset is a convention, the date is a midpoint, and the window closes faster than it opens. The pregnancy calculator and due date calculator apply Naegele’s rule with a cycle-length adjustment, the ovulation calculator works backwards from the next period, and the baby age calculator handles the corrected-age arithmetic afterwards.