Does Gabriel's initial announcement to Mary suggest the virgin birth?
No, it does not. Mary's question proves that it was not made clear to her that there would be a virgin (i.e. miraculous) conception.
For one glaring reason.
She specifically cites the reason she sees it is impossible: an intention to remain a virgin, even in marriage.
More specifically, even though she is betrothed to Joseph, and thus to go to have children with him, eliminating any mystery of how she will "concieve,"Lk 1:31 she cites a reason at odds with having a child, that is:
How shall this be, since I know not man?Lk 1:34
To 'know' man is a well-known Hebraism, a euphemism for sexual intercourse.e.g. Gn 4:1
So Mary is in fact saying, 'How shall this be, since I do not have sex.'
Clearly, Mary knew that sex was required to concieve children (claims that she didn't know how babies were made are absolutely ridiculous), hence why she brings up sexual intercourse at all in relaton to concieving and having a son.
Thus Mary's question, semantically equates to:
'How is this conception possible, if I am remain to remain virgin?'
The angel's answer is such that it seems to take her question exactly that way. Namely, he provides the means of conception, as she asks for:
Luke 1:35 (ASV)
And the angel answered and said unto her, The Holy Spirit shall come upon thee, and the power of the Most High shall overshadow thee: wherefore also the holy thing which is begotten shall be called the Son of God.
That is, an alternative to 'knowing man.' One wonders, if we are to hold she went on to have children anyway way a) she raised the point at all or b) Scriptures nowhere tell Mary to not have relations, nor Joseph.
Since going on to have children is incompatible with her question, we must conclude, as the early Church believed, that she intended to remian virgin.
To refine this even further, specifically, Mary must mean by 'man' Joseph (even though we know it is a euphemism, and 'man' has no referent—in marriage it certainly does), to whom she was betrothed (and at that time, for all intents and purpsoes, married). 'Knowing' another man is absolutely out of the question in marriage, obviously. So Mary's question actually means, 'How shall I concieve this child, if Joseph and I do not have sexual relations?'
None of this makes sense if:
The early Christian Church was unanimous also that Mary didn't have other children, and understood it this way.1
1 Jerome, The Perpetual Virginity of Mary, 18; Gregory of Nyssa, On the Holy Generation of Christ, 5; Augustine, Sermons, 186; cf. Summa Theologiae, III.28.3