Petra is the [strong, grounded, forgotten classic] female form of Peter, meaning "rock" from Greek petros and Latin petrus, and unlike the pairs Henry and Henrietta, or Julian and Julia, Peter and Petra are not as obvious as counterparts. Today's baby name has also had a wealth of variants over the years, such as Perrin, Perrine, Peta, Petronilla, Peronelle, Petrina, Petronia (used by Anne Rice), Pierina, Pernilla, Pella, Petroula, Petruna, Pernille, Peterina, Peternella, Pierette, Petria, Petrova and Petronella. Pier has sometimes been used for girls due to the 1950's actress Pier Angeli.
Famous namesakes: Perenelle Flamel, the wife of alchemist Nicholas Flamel; Perina, the character in the fairy tale of the girl sold with the pears by Italo Calvino; the supposed daughter of St. Peter, named Petronilla, who was made the patron saint of dolphins even though nothing was known about her; Petronella de Meath, the first woman executed for witchcraft in Ireland. Literary: The Weight of Silence, The Chrysalids, Chasing Vermeer, Ender's Game, Posuer. Current namesakes: Petra Nemcova, Petra Haden, Petra Kelly, Petra Ecclestone, Petra Markham, Petra Pau, Petra Yared and Petra de Steur. Film: The Bitter Tears of Petra Von Kant, When Night is Falling, Smiles of a Summer Night. Also the old band Petra, which was Christian rock.
There was also an ancient redstone city in Jordan called Petra. It was rediscovered in 1812 and written about by the poet John William Burgon, who said it was "half as old as time." It is currently considered one of the new "Seven Wonders of the World," along with other marvels like the Great Wall of China and the Taj Mahal. The city's discovery may have contributed to the use of Petra as a given name in the 1800's. Because of this city the name can be seen as less of a feminine variation and more of a place name.
In 2012 there were 118 girls named Petra, on a slight decline from the mid-2000's. It was given to more babies per year in the 1920's, and has been used since at least 1881. The spelling Pernella was given to no more than 9 baby girls between 1904 and 1964, not including some years between those dates. Perina was given to no more than 20 girls between 1911 and 1925, not including several years between those dates. Pierina was given between 1907 and 1948 to no more than 17 baby girls. Other than 1968, the Italian variant Pietra has only been used since 2002, not given to more than 11 baby girls. Petria was only used in 1941 on 5 or more girls (as I've said before, the SSA doesn't release the information unless the name was used on 5 or more kids per name). Perrine was only used in 1919 and 1980. Petronella has been used since at least 1884 on no more than 19 girls a year. Petronia was only used between 1919 and 1984 for a total of 7 years, on no more than 10 girls in a year. Petronilla was used between 1907 and 1935 for a total of 10 years, no more than 7 times in a year. Similar international spelling Petronila was used 29 times total. Petrina was used between 1900 and 2011, and 1971 marked the height of its popularity with 99 births that year, making it the 2nd most popular variant.
Despite the low numbers above, Petra did rank in the U.S. every year between 1880 and 1951. It does not rank now. Very few alternate spellings and other variants have been used in the U.S. Petra is the 2nd most popular name in Croatia and very popular in Hungary. It is also common in Germany and the Czech Republic, among other European places, making Petra a very translatable name.