Now the birth of Jesus Christ took place in this way. When his mother Mary had been betrothed to Joseph, before they came together she was found to be with child from the Holy Spirit.
And her husband Joseph, being a just man and unwilling to put her to shame, resolved to divorce her quietly. But as he considered these things, behold, an angel of the Lord appeared to him in a dream, saying, “Joseph, son of David, do not fear to take Mary as your wife, for that which is conceived in her is from the Holy Spirit. She will bear a son, and you shall call his name Jesus, for he will save his people from their sins.”
All this took place to fulfill what the Lord had spoken by the prophet: “Behold, the virgin shall conceive and bear a son, and they shall call his name Immanuel” (which means, God with us).
When Joseph woke from sleep, he did as the angel of the Lord commanded him: he took his wife, but knew her not until she had given birth to a son. And he called his name Jesus. (Matthew 1:18-25, ESV)
The first gospel has very little to say about the miraculous conception of Jesus. It does not even definitely state anywhere that Mary was sexually pure.
The Greek word translated virgin is parthénos which has a sexual connotation to it but in the context of marriage. It is a rare word even in non-biblical literature, but was used in the Greek translations of the Hebrew Scriptures to translate the verse from Isaiah 7 that Matthew quotes. The problem is that Isaiah 7 is written about Isaiah’s wife who most definitely was sexually active since they had children.
That being said, it is quite clear that Matthew does intend for us to understand that the conception was miraculous. He states quite plainly that the child is “from the Holy Spirit” implying that the pregnancy was not caused by biological means.
There is a certain ambiguity in Matthew’s language, intended to direct to the reader to a conclusion without stating it clearly. God goes to an extreme to protect Mary and her unborn child by sending an angel to her betrothed husband Joseph, and the reader cannot help but conclude that there must be something truly unique about her pregnancy.
It is left to the other gospel writer who records this conception to clarify the nature of the conception.