Here’s a poignant insight from Martyn Lloyd-Jones regarding Christmas:

What is taught in Scripture is not that Mary had been either born or made sinless, but that portion of Mary, that cell out of Mary that was to be developed into the body of the Son of God was cleansed from sin; and that only.

So Mary remained sinful, but this portion that she transmits to her Son has been delivered, set free from sin; and it is to that that the Son of God is joined. That is the human nature He takes unto Himself. It is a miracle, of course; and we are told specifically that it is a miracle. It was because she did not realize that a miracle was to happen that Mary stumbled at the announcement of the Archangel Gabriel, and his reply to her is: “Do not try to understand this. The power of the Highest shall overshadow you, the Holy Ghost will come upon you, you are going to conceive of the Holy Ghost” – who has power to work this cleansing so that the body of the human nature of the Son of God who is to be born of you shall be entirely free from sin.

So we reject the so-called doctrine of the “Immaculate Conception”; but we assert with all our power the doctrine that the human nature of the Son of God was entirely free from sin.