How much does my baby weigh?

Less precise than people expect, and that's true of every scanner and every formula — not just this tool. Ultrasound estimates weight indirectly, from measurements of the head, abdomen, and femur, and the published Hadlock models carry an inherent error of roughly ±10–15% even in experienced hands. Near term that can mean the real birth weight is several hundred grams — close to a pound — above or below the estimate. That's why clinicians read EFW as a range and pay far more attention to the trend across scans than to any single number. Treat the result here the same way: a ballpark, not a weight on a scale.

They're the four standard biometry measurements. HC is head circumference, the distance around the head. AC is abdominal circumference, the distance around the belly at the level of the liver — the measurement that influences the weight estimate most. FL is femur length, the thigh bone. BPD is biparietal diameter, the side-to-side width of the head. Most US reports list them in millimetres (some in centimetres — 1 cm is 10 mm), often alongside a gestational-age equivalent for each. This tool needs HC, AC, and FL, and can also use BPD if your report includes it.

Small differences are expected. There are several published Hadlock models (and other formulas entirely, like Shepard or INTERGROWTH), and different ultrasound machines are configured with different ones. Rounding matters too: a measurement recorded as 33.4 mm versus 33 mm shifts the output slightly, and some software averages multiple takes of each measurement before computing. If the gap is small — a few percent — that's formula and rounding noise. Whatever the difference, the number your sonographer reported is the real one for your care: it was computed from the exact measurements your clinic recorded and is the figure your provider tracks.

Not by itself. The medians in the table are just the middle of a wide healthy range — half of all babies weigh less than the median and half weigh more, and perfectly healthy babies sit well above and below the broad bands shown here. On top of that, the estimate itself carries a ±10–15% margin, which alone can move a baby from one side of the median to the other. A single snapshot also can't show the thing that actually matters clinically: growth over time. So don't attach a label to one number — ask your provider what your baby's growth curve shows across scans. That conversation, not this calculator, is the real answer.

The Hadlock regression models published in the American Journal of Obstetrics & Gynecology in 1985, which remain the most widely used EFW formulas in the world. With HC, AC, and FL, it applies the three-parameter model: log10 EFW = 1.326 − 0.00326·AC·FL + 0.0107·HC + 0.0438·AC + 0.158·FL, with measurements in centimetres (your millimetre entries are converted automatically). If you also enter BPD, it switches to Hadlock's four-parameter model, which uses all four measurements. The week-by-week medians in the reference table come from Hadlock's 1991 in-utero fetal growth standard.

Yes. This calculator runs entirely in your browser: the measurements you type are converted and computed on your own device and are never sent to a server, stored, or shared — close the tab and they're gone. We treat pregnancy data as sensitive by design, so there is nothing to delete and no account involved. The result is general educational information based on published formulas; it isn't medical advice or a diagnosis, and it can't replace the sonographer's report or the provider who reads it.