Anna Freud (1895 - 1982) is one of the founders of psychoanalytic child psychology. She was influenced by her father, Sigmund Freud, but her work focused more on the importance of the Ego and its role in striving to strike a balance between the Id's desire for pleasure, and the Superego's moralistic demands. She expounded on the concept of Defense Mechanisms as the Ego's means for dealing with anxiety.
Anna Freud introduced the concept of Developmental Lines, which emphasize the nature of child development as a continuous and a cumulative process, and the idea that it is possible to move forward (progress) or backwards (regress) along the continuum.
Anna believed that it may sometimes be necessary for an individual to regress for a brief period of time in order to deal with some overwhelming challenge. Once the person overcomes the challenge, he/she may again move forward.
For example, an only child who has successfully mastered toilet training may seem to "forget" his training when faced with the pressure of an impending sibling. Once he has adjusted to the new member of the family, then the issue will be resolved and he will be ready to move forward again.