US women, Clothes: dress and suit
US standard clothing sizes were developed from statistical data in the 1940s-1950s. These are on average 6 sizes larger than the original EU standard. So, for example, a size 12 on the old standard would today be described as a size 6, while a size 6 on the old scale would be what is today known as size zero. Read more 
The European Union has produced a standard EN 13402 intended to replace existing standards in the member countries. This is not yet in common use. The United Kingdom has an existing standard for women's clothing, however this is rarely followed by manufacturers as it defines sizes in terms of hip and bust measurements only within a limited range. See table of sizes. This has resulted in variations between manufacturers and a tendency towards vanity sizing. In the US there exists a US standard clothing size.
Read more
Clothing