Margrethe II (Margrethe Alexandrine Þórhildur Ingrid, born 16 April 1940) is Queen of Denmark. Having reigned as Denmark's monarch for over 50 years, she is the world's only current queen regnant and the longest-serving current female head of state. Born into the House of Glücksburg, a cadet branch of the House of Oldenburg, Margrethe is the eldest child of King Frederick IX and Queen Ingrid. She became heir presumptive to her father in 1953, when a constitutional amendment allowed women to inherit the throne. Margrethe succeeded her father u…