Margaret Hilda Thatcher , Baroness Thatcher LG , OM , PC , FRS ( born Margaret Hilda Roberts , 13 October 1925 ) was the first female Prime Minister of the United Kingdom , a post she held from 1979 to 1990 . Margaret Hilda Thatcher , Baroness Thatcher , ( nÃ e Roberts ; born 13 October 1925 ) is a former Prime Minister of the United Kingdom who served from 1979 to 1990 .