It was the world of 'miuchi' during the period of regents and their families or a person who became the 'miuchi' of the emperor who became a regent and had no notion of a legal son.
摂関時代は「ミウチ」の世界であり、それ故に摂関家自体、天皇の「ミウチ」になった者が摂関となるのであり、嫡男などという概念は無い。