Why is birth weight plotted on a bell curve?

Statistics. Most human development parameters exist with a range of data over the whole population, and a bell curve is usually the shape of such data. So plotting birth weight will produce, for a given age, a range of weight, height, head size, length, etc. A bell curve.
Normal distribution. A bell-shaped curve, also called a normal distribution, shows the variation of a measured value around a mean, or average. In the US, the average baby weighs about 7 1/2 pounds. Birth weights follow this pattern with most babies weighing close to the mean. This means that there are lots of babies between 6 and 8 pounds but relatively fewer 4 and 10 pounders.