In the time of the Virgin Mary, the expectation of the Romans might have come from the great oracle reported by the Latin poet Virgil (70-19 BC) in the 4th Eclogue of his "Bucolics":
"Here is the last period marked by the oracle of the Sibyl of Cumae: the long series of centuries begins again. Here comes the Virgin, and the reign of Saturn. Here is a new race descending from heaven. A newborn child under the reign of Emperor Augustus will eliminate the iron race and create a generation of gold everywhere."
The Virgin Mary, in whom the Son of God descended, probably had not heard of that oracle, but Jesus, who was born in the reign of the Emperor Augustus, did transform the iron of oppression into love, which gold symbolizes.
In several religious shrines around the world (like Longpont, Nogent-sous-Coucy, and Chartres, to mention only those located in France), surprisingly enough a certain "Virgini Pariturae," a "Virgin who must give birth," was already venerated before the coming of Christ.