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MasterMath

Estimated Fetal Weight

With a biparietal diameter of 8.5 cm, a head circumference of 31.5, an abdominal one of 29 and a femur of 6.2, the estimated weight is around 2,100 grams. Apply the Hadlock formula that scanners use.

Estimated weight

—

Estimated weight—
With the scan's margin—
In pounds—
Width of the margin—
Formula applied—

How this was worked out

    Indicative result. This is an estimate with a margin of around 15%, not a measurement. Only the person who performed the scan and knows the case can interpret it: two babies with the same measurements can differ by half a kilo.

    The formula

    log₁₀(weight) = 1.3596 − 0.00386·AC·FL + 0.0064·HC + 0.00061·BPD·AC + 0.0424·AC + 0.174·FL

    Where it comes from

    Hadlock published the formula in 1985 and it is still the most used, because it combines four measurements taken as a matter of routine: the width of the head, its circumference, the circumference of the abdomen and the length of the femur. The abdominal circumference carries the most weight in the result, because it reflects fat tissue and the liver, which is where growth shows up first.

    How to work it out by hand

    1. Note the four measurements from the scan in centimetres
    2. Substitute them into the Hadlock formula
    3. The result is a logarithm: raise ten to that figure
    4. Apply the 15% margin to get the real interval

    What is worth knowing

    The margin is the point of this page. 15% of three kilos is 450 grams either side, nearly a kilo of width, which means an estimate of «four kilos» is compatible with a 3,400-gram baby. When that figure is used to decide on an induction or a caesarean, it is worth remembering that a third-trimester scan does not tell a large baby from a normal one well: studies find the error grows precisely at the extremes, which is where the decisions get made.

    Frequently asked questions

    How wrong can a scan be about weight?

    Around 15%, and more at very high or very low weights. It is an interval, not an exact figure.

    Which measurement matters most?

    Abdominal circumference. It reflects recent growth and moves the result the most.

    Does it tell me if the baby is macrosomic?

    It gives a hint, not an answer: scans identify weights above four kilos poorly, and that is exactly the case they get used for.

    Can I use a different formula?

    There are several — Shepard, Warsof, Campbell — and they give similar results. Hadlock's four-parameter one is the most widespread.