The number of women studying in theological seminaries tripled from 1972 to 1979, with women now totaling more than 20 percent of enrollment, according to the new Yearbook of American Churches.

The annual compilation of statistics on organized religion in the United States and Canada shows a slight continued decline in membership in main-line Protestant denominations. But moderate growth in conservative evangelical bodies has kept the overall increase in church membership -- seven-tenths of a percent -- almost on a par with population growth.

The statistics in the volume published this week are good for the year ending Dec. 31, 1978. Indications are that the numbers of women seminarians has continued to increase since that time.

Locally, women made up 23 percent of the student body at Virginia Theological Seminary, an Episcopal school in Alexandria, for the academic year just ended. At the United Methodist Wesley Theological Seminary in Northwest Washington, women made up 36 percent of the student body.

An analysis of church membership statistics, included in the yearbook, indicates that 40 percent of American Protestants now belong to a different denomination than the one in which they were raised.

The study, done by C. Kirk Hadaway of the Southern Baptist Convention's Home Mission Board, indicated that theologically conservative groups with aggressive evangelization programs have benefited most from such switching.

Hadaway points out that while liberal denominations such as the United Church of Christ and the Episcopal Church have gained many new members through conversions and transfers, they appear to lose even more members who drop out entirely.

The highest rates of loss in 1978 were sustained by the United Presbyterian Church (1.6 percent), the Lutheran Church-Missouri Synod (1.57 percent) and the United Church of Christ (.93 percent).

Despite predictions by traditionalist Episcopalians that members would leave following changes in the prayer book and the decision to ordain women to the priesthood, the Episcopal Church's rate of loss in 1978 was only .12 percent.

The fastest-growing churches were the Presbyterian Church in America (11.09 percent), the Free Methodist Church of North America (6.2 percent) and the Church of God of Cleveland, Tenn. (3.91 percent).The Southern Baptist Convention, the nation's largest Protestant body, reported a gain of .87 percent, just slightly more than the U.S. population growth rate of .8 percent for the year.