Margaret Thatcher became the first female Leader of the Conservative Party and Leader of the Opposition after winning the 1975 leadership election, the first Conservative leadership election where the post was not vacant. A rule change to enable the election was largely prompted by dissatisfaction with the incumbent leader, Edward Heath, who had lost three of four general elections as leader, including two in 1974. After announcing her first Shadow Cabinet in February 1975, she reshuffled it twice: in January and November 1976. Minor subsequent changes were necessary to respond to various circumstances. Thatcher's Shadow Cabinet ceased to exist upon her becoming Prime Minister following the 1979 general election.