Reverse Due Date Calculation: Working Backwards From Gestational Age

What the calculation does

Standard due-date arithmetic runs forwards: take the first day of the last menstrual period and add 280 days, the length of a 40-week pregnancy under the conventional model. The reverse calculation runs the same model backwards. If a scan or clinician has stated a gestational age on a particular date, subtracting that interval recovers the LMP the model implies, and adding 280 days to it gives the estimated due date.

This is genuinely useful when the actual LMP is unknown, uncertain or has been superseded. Irregular cycles, no clear last period, or a dating scan that revised the original estimate all leave you with a gestational age and no matching LMP. The reverse calculation fills that gap arithmetically.

Use this guide with the right tool: Open the Reverse Due Date Calculator for the estimated due date and LMP implied by a current gestational week and day. If the question shifts, compare it with the Pregnancy Due Date Calculator or the Gestational Age Calculator.

Implied LMP is not necessarily the real one

The number this produces is a model artefact, not a historical fact. If the gestational age came from an ultrasound measurement, the implied LMP is whatever date makes the standard model agree with that measurement — which may differ from the actual last period by days or more, particularly with irregular or long cycles.

That is not a flaw; it is the point. Dating is deliberately re-anchored to the more reliable measurement. But the implied LMP should be labelled as implied, and it should not be entered anywhere as though it were an observed date.

Why 280 days, and what it hides

The 280-day figure assumes a 28-day cycle with ovulation on day 14, which makes gestational age about two weeks longer than the time since conception. It is a standardised convention rather than a biological constant, and it is why gestational age and fetal age differ by roughly a fortnight.

Actual gestation length varies substantially between pregnancies. The due date marks the centre of a distribution, not a deadline — most births occur in a window of several weeks around it. A calculator can be exact about the arithmetic and still be describing an estimate.

What this cannot replace

This is an informational arithmetic tool. It cannot assess a pregnancy, adjudicate between conflicting dating methods, or account for clinical factors that change management. Where a scan-based date and an LMP-based date disagree, the decision about which governs is a clinical one with established criteria behind it.

Use the calculator to understand how a date was derived, to check that two numbers you have been given are consistent, or to see the effect of a revised gestational age. Take anything that affects care to the midwife, obstetrician or clinician managing the pregnancy.

Worked example

The shape of the calculation stays the same with your own figures. What changes is the source of the rule, so check that before treating the number as settled.

Common mistakes to avoid

• Recording an implied LMP as an observed date. It is derived from the model. Label it as implied and keep any known actual date separately.
• Assuming the due date is a deadline. It is the centre of a distribution. Births occur across a window of several weeks around it.
• Overriding clinical dating with calculator output. Ultrasound and clinical assessment take precedence. This tool explains the arithmetic; it does not decide the date.

Use the right calculator

Start with whichever matches the information you already hold. The other two are there for when the question shifts.

Pregnancy & PediatricReverse Due Date Calculatorthe estimated due date and LMP implied by a current gestational week and day

Pregnancy & PediatricPregnancy Due Date Calculatoran estimated due date from the last menstrual period and cycle length

Pregnancy & PediatricGestational Age Calculatorgestational age from the first day of the last menstrual period

Frequently asked questions