Size matters

When considering outcomes of pregnancy, size at birth is among the most important characteristics of a successful pregnancy. In addition to duration of pregnancy and the qualitative development of the fetus, the anthropometric size of a newborn baby is of considerable significance. Long before the relatively modern concept of gestational age was well understood, medical personnel recognized and recorded the size, particularly the weight, of newborn babies, and both mortality and morbidity were correlated to birth size. This persists even today, with terms such as “Low Birth Weight” infants, which do not incorporate the concept of duration of pregnancy, as an important descriptor in the public health arena. Similarly, among the first questions asked by parents, is “How much does my baby weigh?,” a reflection of the importance of size in the common understanding of pregnancy.

As a practical issue, clinicians pay most attention to weight, length and head circumference, in part because both scales and rulers are easily available and accurate. Weight is particularly emphasized, in part because the measurement of weight is particularly insensitive to inter-observer measurement error. Other measures, such as surface area, BMI and weights raised to various powers (e.g. wt0.75, wt2, wt/length2) are also in common use, but require more difficult calculation or measurement. The close attention paid to weight is more a reflection of convenience, than biological importance, as other measures may more closely relate to matters of biological importance.