I'm just assuming it's not "growth" you're looking for, but that you're question is on how a child's measurement is expressed.

Contrary to my first note above, it's "infant length", as the child is measured lying down.
Here's a confirmation:

Length ofchildren less than 2 years old is measured lying down, while standing height is measured for children age 2 years or older.
www.who.int/entity/childgrowth/training/module_a_introducti...

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Note added at 22 mins (2008-12-16 11:43:15 GMT)
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Also:

Length vs. Height
By convention, in infants 2 years and younger, we measure length, in children over 2 years we measure height. Length is measured with the infant lying down, whereas height is measured with the child standing up. The reason for making the distinction is that length values are slightly longer that the height values.
http://www.aucuba.net/ImmunoDocs/MeasurementsHeight.aspx