The Ivy League is considering an expansion from its traditional eight schools to a 10-team league, with Army, Northwestern and Navy the most likely candidates for membership. However, none of these institutions has been formally approached on the subject, according to league officials.

The idea of expansion appears to have come about shortly after the Ivy League was demoted from the major college group, Division I-A, to Division I-AA by a vote of the National Collegiate Athletic Association on Dec. 4. During a special convention of the N.C.A.A., the majority of Division IA institutions approved attendance and stadium-size requirements for membership in Division I-A that the majority of the Ivy League teams could not meet.

Thus the entire league was forced into Division I-AA at the same time that I-A was cut from 137 teams to fewer than 100. David McLaughlin, president of Dartmouth College, an Ivy member, said today: ''I'm sure it's prompted somewhat by the N.C.A.A. action. It's time to really sit back and take several months to think together on what the league should be and how it fits within the N.C.A.A. rules and whether the league should be expanded.''

The N.C.A.A. reduced its Division I-A football membership when it required that each team in this major category have an average home attendance of 17,000 for the last four years or a home arena that seats at least 30,000 people. Only Yale among the Ivy teams met both of those reguirements.