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Is there any US/UK English difference in the spellings "birth weight" and "birthweight"? In scientific journals, I have found usage of both spellings.
E.g., Overall birth weight is not different for physically active women, though physical activity may reduce the risk for large-for-gestational age newborns.
As far as I can see, the correct usage is "birth weight" as two separate words, although the meaning of "birthweight" is immediately obvious and would be accepted by most people. This is an example of a compound word https://en.wikipedia.org/wiki/Compound_(linguistics), a phenomenon which is quite common when two words are repeatedly used together. Many English words are compound words: the process is that if the compound appears in print enough times it makes it into the dictionary and then it's "officially" a word.
As an aside, William Shakespeare gave a great number of words to the English language, and many of those are compound words. eg http://mentalfloss.com/article/48657/20-words-we-owe-william-shakespeare
