What prompted Le'ah to offer her maidservant to Ya'akov?
Tosfos ha'Shalem (1), citing R. Efrayim, Ramban: The Imahos were Nevi'os, 1 and knew that they would bear Ya'akov twelve tribes. Consequently, Le'ah wanted that the majority of the tribes should come from her or from her maidservant.
Why did Ya'akov accept Le'ah's offer?
Ramban #1: Knowing that Le'ah's prediction was correct, Ya'akov conceded to her request.
Ramban #2: Because, since the fourth generation was destined to return to Eretz Cana'an, and Avraham and Yitzchak did not have many (eligible) children, Ya'akov opted to marry many wives, in order to fill the land upon their return.
Asarah Ma'amaros (Chikur Din 3:18): Leah put Zilpah in place of herself. Yakov did not know. 1
Surely, he realized before Asher was conceived! Perhaps once he saw that one of his sons was from Zilpah, he realized that she is one of the four wives he must have (PF).