Complex sentence: It was most popular in the UK in the 1980s, and in the US during the 1990s, when over 2,000 girls were named Carly each year between 1991 and 1998.
Split into simpler sentences: It was most popular in the UK in the 1980s, and in the US during the 1990s. Over 2,000 American girls were named Carly each year between 1991 and 1998.