From the experts:
Ever since the enormous German cannon was dubbed by Allied soldiers "Big Bertha" in World War I, this name hasn't worked for a sweet little baby girl. But this was not always so. Hard as it might be to imagine now, Bertha was a Top 100 name until the 1930s, and in the 1880s was the seventh most popular name in the land--the equal of Joseph.
Bertha, aka Berchta, was a Teurtonic goddess borrowed from Norse mythology, regarded in her earliest incarnations as a beautiful and benevolent spirit. Bertha is an ancient Germanic name attached to the 6th century Saint Bertha of Kent, who was instrumental in the spread of Christianity, and it was the name of both the mother and daughter of Charlemagne.
In literature, Bertha Mason is the first wife of Rochester in Jane Eyre, and a character in Edith Wharton's The House of Mirth. The French version Berthe is represented by the French Impressionist painter Berthe Morisot. All in all, though, modern parents might do better to consider the Polish version, Berta.