Standard, Population & Customised fetal size charts 7 – maternal weight

Underweight women have smaller babies and overweight ones larger. Compared with ethnicity (click here and here) customising by weight should be straightforward. But it’s still unlikely to improve detection of adverse outcomes.

Outside a relatively narrow band, BMI 18-25, weight variation is not healthy. Undernourished women, smokers and users of other substances, have smaller babies, but also more dead and brain damaged ones. Those who are underweight because their mothers had been undernourished during their own pregnancy, probably also have higher rates of adverse outcomes.

Similarly the overweight population includes women who are over eating, and women with diabetes or pre-diabetes, whose babies are large in an unhealthy way. They also have more stillbirths, birth injuries and other adverse outcomes.

This is why, at the extremes, customising on maternal weight is harmful. It leads us to tell an overweight diabetic mother with a BMI of 35 that her macrosomic baby was “normal for her weight”, or an underweight woman with a malabsorbtion syndrome, that her small baby was “normal for her”!

Customising on weight within the relatively narrow band of “healthy” weight variation might make sense, but in practice variation in fetal size within this normal band is small.  Enthusiasts for customisation such as the Perinatal Institute (click here) recognise this and their GROW customisation software limits the adjustment on the basis of weight to the central BMI zone. However, presumably because they wish to “explain” more birth weight variation, they customise up to BMIs of 30, the borderline between “overweight” and “moderately obese”. This is not a healthy weight.

Next customisation by height (click here)

Jim Thornton