How a Due Date Is Worked Out

A due date is presented with a precision it does not have. It is a single day derived from an average, and the overwhelming majority of babies do not arrive on it.

How the date is calculated

The due date calculator supports the two standard methods.

From the last menstrual period (LMP). The traditional approach adds 280 days — forty weeks — to the first day of the last period. It assumes a 28-day cycle with ovulation around day 14, which is why the tool lets you adjust the cycle length: a consistently longer or shorter cycle shifts the date accordingly.

From a known conception date. Where that is known, the calculation is 266 days — thirty-eight weeks — from conception, since the LMP method effectively counts two weeks before conception occurred.

That two-week offset is the source of the perennial confusion that pregnancy is "counted from before you were pregnant". It is a convention, adopted because the last period is a date most people know and conception usually is not.

Why the date is an estimate

Cycle length varies between people and between cycles. Ovulation does not reliably occur on day 14. And the length of a pregnancy itself varies naturally by a couple of weeks either side.

The practical consequence is well known to anyone who has been pregnant: only a small minority of babies arrive on their due date, and a spread of a fortnight either side is entirely ordinary. A dating scan in early pregnancy is generally more accurate than either calculation, and where a scan disagrees with the LMP date, the scan usually takes precedence.

What it is actually for

Despite the imprecision, the date does real work. It anchors gestational age, which is what antenatal appointments, screening windows and scan timings are all scheduled against — and several of those have windows measured in days rather than weeks.

It also sets the frame for decisions later in pregnancy about monitoring and, where relevant, induction — all of which are clinical matters and belong with your midwife or doctor.

A better mental model

Think of it as the middle of a range rather than an appointment. "Due around the end of March" is a more honest statement than a single date, and it manages expectations better — particularly with employers, family, and anyone planning to visit.

Practically, being ready a couple of weeks before the date and unbothered a week or two after it removes a great deal of unnecessary stress.

Where the tool stops

The calculator does arithmetic on the dates you give it. It cannot account for irregular cycles, for uncertainty about the LMP, for a scan that has revised the date, or for anything clinical about the pregnancy.

Your midwife or doctor holds the date that matters — the one your care is actually scheduled against — and where the two differ, theirs is the one to use. This is general information rather than medical advice, and anything about how a particular pregnancy is progressing belongs with them.

Gestational age, and why the weeks matter

The date's real job is anchoring gestational age, which is how the whole of antenatal care is organised. Screening tests have windows measured in days, scans are scheduled at particular weeks, and the interpretation of several results depends on knowing gestation accurately.

That is why a dating scan carries more weight than a calculation: getting the gestational age right early affects everything scheduled afterwards, and a date revised at the first scan is the one your care will follow.

Term is a range, not a day

Obstetric practice divides the end of pregnancy into bands rather than treating forty weeks as a target — early term, full term, late term and post-term each cover a span of weeks, and the majority of babies arrive across that whole range.

Knowing that helps with the last fortnight, which is otherwise spent feeling overdue from the moment the date passes. A baby arriving after the date is usually not late in any clinical sense.

What to plan around it

Practically: be ready a couple of weeks early, and expect to still be waiting a week or two after. Tell people a range rather than a date. And arrange anything time-critical — handovers at work, visitors travelling — with slack in both directions.

Decisions about monitoring or induction as the date passes are clinical and belong entirely with your midwife or doctor, who will be working from your own dating and circumstances rather than from an arithmetic estimate.