Covering the Hair after Marriage
The obligation of a woman to cover her hair takes effect only after the first night since it is from that point on that she has the status of one who has been intimate with her husband. She does not have to cover her hair immediately after chuppah and yichud since at that point she does not yet have the status of one who has been intimate with her husband. The reason is obvious, since as a matter of marriage law, there is no distinction between an arusah (betrothed) and a nesuah (fully married), rather the obligation to cover her hair is dependent on whether or not she has been intimate with her husband.
(Responsum from R. Moshe Feinstein, dated 15 Elul 5745, to R. Aharon Tendle).