Yes, the dictionary definiton could be modified to read "having a child, children, or young"... It is correct to say "pregnant with triplets" (not "for three children")
And again the dictionary definition of "noun phrase" could be sharpened to say "one or more modifiers". Or if you want a formal grammar definition, "zero or more modifiers or determinants" (a phrase can consist of a single word though in context it is often understood mean more than one word).
This kind of detail is often omitted, as it is assumed that the reader will fill in the details using "common knowledge" or "context and experience". And it is why universities have to teach mathematics students how to write definitions and proofs that don't depend on common knowledge. A dictionary might define "prime number" as "A number which can't be divided into equal parts". A student might say "A prime number is a number which has no factors except for 1 and itself". But a mathematician might say it is "a positive integer that has exactly two positive whole number factors".
Another example is "rectangle". A mathematical defintion often includes squares as a type of rectangle, but general use tends to imply that the shape is not a square.
In a mathematical definition, you can't assume common sense, but dictionaries do.
