One in twenty
ACOG puts it plainly: only about 1 in 20 women give birth on their due dates.
So the date on the paperwork is not a prediction of the day. Nineteen times out of twenty it will be wrong, and often by more than a week. Anyone who has had a nursery ready three weeks early, or a bag packed for a fortnight past the date, knows this in their bones.
What the date does is anchor everything else. Gestational age is counted from it, growth is tracked against it, and the timing of every scan and screening test through the pregnancy hangs off it. The due date is a coordinate, not a forecast.
That reframing changes how much weight to put on it. A coordinate needs to be consistent. It does not need to come true.
Where the number comes from
The starting point is the first day of the last menstrual period, and the arithmetic adds 280 days, which is 40 weeks. This is Naegele's rule, and it is what a due date calculator does when you give it that date.
Two assumptions sit inside it. First, that the cycle is 28 days. Second, that ovulation happened on day 14. Neither is true for a great many people, and a longer cycle shifts the real date later than the rule says. Some calculators let you enter cycle length to correct for this, which helps, though it still assumes you know it.
The bigger problem is the input. Plenty of people do not know the first day of their last period to the day, and the whole calculation inherits that uncertainty.
Which is why the number from the calculator is a first draft and not the final one.
When the ultrasound overrules the calendar
ACOG describes what happens next. When the date of the last period is uncertain, an ultrasound in the first trimester is used to estimate the due date instead.
The measurement is the crown-rump length, from the top of the head to the bottom of what will become the buttocks. A formula turns that length into a gestational age. Before about seven weeks the crown and rump cannot be distinguished, so the greatest length of the embryo is measured instead.
Early ultrasound dating is accurate because embryos at that stage grow at a remarkably consistent rate. Later in pregnancy, size varies far more between babies, so a scan at 30 weeks tells you much less about age than a scan at 9 weeks does.
If the scan and the calendar disagree, the clinician will usually go with the scan, and the due date on your notes may change. People find that unsettling. It is the opposite: the earlier date was the guess, the scan is the correction.
IVF pregnancies are the exception, and a clean one. The date of transfer is known precisely, so the due date is calculated from that rather than from a period or a scan.
The dates that actually matter
Since the due date itself is unreliable, the useful thing is to know the window around it.
Full term is 39 weeks 0 days to 40 weeks 6 days. Early term runs from 37 weeks 0 days. Late term is 41 weeks, and post-term begins at 42. Those boundaries drive decisions about monitoring and about whether labour is induced, which is why the date needs to be consistent even though it will not be hit.
For planning, work backwards from the window rather than the date. Thirty-seven weeks is a more honest "be ready by" point than forty. The date difference calculator will tell you how many weeks and days you are from any of these, and the business days calculator is the one for working out when to stop taking new clients or when leave should start, since those run on working days rather than calendar ones.
What I would stop doing with the date
Stop telling people the date. Tell them the month, or "late October". The date invites a countdown that will almost certainly be wrong, followed by a fortnight of messages asking whether anything has happened yet.
Stop treating a changed date as a problem. A date that moves after an early scan has been corrected, not broken.
Stop reading anything medical into the day itself. Being a week past it is ordinary. Being two weeks past it is a conversation with the midwife or doctor, and that conversation is why the date exists.
None of this is medical advice, and the person to ask about your own pregnancy is the person looking after it. What a calculator gives you is the arithmetic. What it cannot give you is the scan.
Questions people ask
By adding 280 days, or 40 weeks, to the first day of the last menstrual period. This is Naegele's rule and it assumes a 28 day cycle with ovulation on day 14. When the period date is uncertain, a first-trimester ultrasound measuring crown-rump length is used to estimate the date instead.
As a prediction of the actual day, not very: ACOG notes only about 1 in 20 women give birth on the date. Its job is different. It fixes gestational age so growth can be tracked and tests scheduled, and for that it only needs to be consistent, which is why an early scan may revise it.
Because the scan is more reliable than the calendar. Early embryos grow at a very consistent rate, so measuring one gives a better estimate of age than counting from a period date that may itself be uncertain. A revised date after an early scan is a correction rather than a cause for concern.
Full term is 39 weeks 0 days to 40 weeks 6 days. Early term starts at 37 weeks, late term at 41, and post-term at 42. Those boundaries, rather than the single due date, are what drive decisions about monitoring and induction.
IVF uses a different and more precise method. The date of embryo transfer is known exactly, so the due date is calculated from that rather than from a period date or a scan, and it does not carry the cycle-length uncertainty that Naegele's rule does.
The window, not the day. Thirty-seven weeks is a more honest be-ready-by point than forty, because a baby arriving any time from then is considered early term. Work backwards from that for leave, handovers and anything else with a deadline.