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Ultrasound Due Date Calculator Guide
This dates a pregnancy from a crown-rump length measurement — the length of the embryo from head to bottom — using the Robinson-Fleming formula, which is the standard for first-trimester dating.
Gestational age (days) = 8.052 × √(CRL × 1.037) + 23.73, with CRL in millimetres.
A 20 mm CRL gives 60.4 days — that's 8 weeks 4 days.
Due date = scan date + (280 − gestational age).
Why the First Trimester Is the Best Time to Date
Early embryos grow at a remarkably consistent rate. Genetics, nutrition and everything else that later makes babies different sizes hasn't kicked in yet, so size maps to age tightly. A CRL between 7 and 14 weeks dates a pregnancy to within about five days.
After 14 weeks that consistency falls apart. By the third trimester, size-based dating can be off by three weeks in either direction, because by then babies are simply different sizes. This is why a late scan never changes your due date — a large baby at 32 weeks is a large baby, not an older one.
The Validated Range Matters
Robinson-Fleming was built on measurements from roughly 10 to 84 mm. Push it outside that and you're extrapolating a curve beyond its data. Below 10 mm the embryo is small enough that a fraction of a millimetre of measurement error moves the date substantially; above 84 mm the relationship flattens and other measurements take over.
This calculator flags when your input falls outside the range rather than quietly returning a number that looks authoritative.
The Scan Usually Wins
If an early scan disagrees with your last-period date by more than about five to seven days, most guidelines say re-date the pregnancy to the scan. It's not that the scan is infallible — it's that the LMP method rests on assumptions about your cycle that are frequently wrong.
Once a dating scan sets the due date, it stays. Later scans measuring growth don't move it, because at that point you'd be measuring size, not age.
Why It Matters More Than It Seems
The due date isn't just about knowing when to pack a bag. It sets the timing of screening windows, decides whether a baby is "early term" or "post term", and drives decisions about induction. Being out by a week changes real clinical choices — which is why the dating scan is one of the more consequential appointments of the first trimester.
If You Don't Have a CRL
Use the due date calculator from your last period, or the pregnancy week calculator if you already have a due date.
This is an estimate for general information, not medical advice. Every pregnancy and every cycle is different, and only your doctor or midwife can give you dates and guidance for your situation. Nothing here replaces professional care.
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