Correction: this council that I mentioned was not a General Council. However, the inclusion of the word “virgin” in the Nicene Creed is pretty authoritative. I suppose you could say it was defined in Council at the Nicene Council, by virtue of its inclusion in the Creed. However, it was also included in the Apostles’ Creed which predates the Nicene Council, so we’re talking about a dogma that has been defined in the earliest of days.
I suppose you could say that the word “virgin” in the Creed only implies that she was a virgin at the time of the birth of Jesus. I would suggest reading the article I linked to above for more information.