Estimated fetal weight calculator

This estimated fetal weight calculator applies the classic Hadlock four-parameter equation to standard obstetric ultrasound biometry: biparietal diameter, head circumference, abdominal circumference, and femur length. The model maps those lengths, expressed in centimetres, through a published log10 polynomial and returns estimated fetal weight in grams, kilograms, and pounds without network calls, sampling, or date-dependent shortcuts. Enter measurements in centimetres or millimetres, optionally set decimal precision, and read back normalized centimetre values, the intermediate log10 result, a formula string, and step lines you can show beside the answer. Treat the output as an educational and engineering estimate, not a clinical diagnosis or substitute for professional obstetric judgment.

What the Hadlock weight estimate tells you

Estimated fetal weight in the Hadlock sense is a regression-derived mass predicted from four fetal size measurements commonly taken on second- and third-trimester ultrasound: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). The four-parameter form used here follows the widely cited log10 model in which the common logarithm of weight in grams is a linear combination of AC times FL, HC, BPD times AC, AC, and FL, plus a constant intercept. Raising ten to that log10 value yields estimated fetal weight in grams.

Missing any of the four lengths always fails because the four-parameter Hadlock model requires BPD, HC, AC, and FL together. Zero, negative, non-finite, or out-of-range measurements after unit conversion also fail with field-named messages so a bad form cell does not become a wrong gram figure.

How biometry fields are read and how results stay reproducible

Optional unit accepts centimetres (default) or millimetres so report sheets that print millimetres can post without pre-dividing by ten; every length shares the same unit on a single request. Optional leading plus signs and scientific notation on numeric strings are allowed when they parse as finite decimals; embedded spaces, hex prefixes, and non-numeric tokens are rejected with a clear message.

After conversion, each measure must lie inside a practical centimetre window that catches inverted unit mistakes without pretending to enforce a gestational-age chart: BPD roughly half a centimetre to fifteen centimetres, HC and AC from two to fifty centimetres, and FL from three tenths of a centimetre to twelve centimetres. Computation builds the log10 polynomial with fixed published coefficients, exponentiates base ten, and derives kilogram and pound companions with a pounds-and-ounces helper for US-style display.

Where a Hadlock EFW calculator earns its keep in education and software

Sonography and obstetric coursework introduce Hadlock estimates when textbooks walk through standard biometry packs and ask learners to convert four lengths into a single gram figure. Product teams call the same contract from scripts to seed deterministic test vectors for maternal-fetal dashboards, to validate user-entered ultrasound fields before related tools run, or to satisfy programming-challenge statements that demand a Hadlock four-parameter estimate rather than a black-box app.

Clinical-engineering demos sometimes show EFW beside measured birth weight purely as a historical regression reference, while reminding readers that real growth assessment depends on percentiles, serial scans, maternal context, and professional interpretation. Treat the output as an educational and engineering estimate: if clinical or research accuracy is required, prefer your institution’s ultrasound package, published growth charts, and a qualified clinician. Pair this tool with other health calculators when you need related anthropometrics, and keep units honest so millimetres and centimetres are never mixed by accident when building forms.