Is "Gestational Diabetes" a kind of "Diabetes"? Two coding systems, two answers.
In SNOMED CT, gestational diabetes is listed as a hierarchical descendant of diabetes mellitus, but in ICD-10-CM, it isn't. I think both organizations are right, and reflect the basic uses for both terminologies. SNOMED CT is all about clinical documentation and inferential logic. So, it makes sense to recognize that gestational diabetes shares many pathophysiologic features with other kinds of diabetes. But, ICD-10-CM is a classification system, and it makes sense that you'd want to report and analyze gestational diabetes in its own bucket.
As long as you are aware of the way the two coding systems are organized, you can get the results you're looking for out of either one. When you're constructing a diabetes value set in SNOMED CT, you might decide to exclude the gestational diabetes subtree, or not. Similarly, if you're building a value set in ICD-10-CM, you might have to include two separate high-level codes... or not!