Excuse me if I say that “placed in the womb” sounds like insemination! Anyway the doctrine of the Immaculate conception is not expressed in physical terms at all. She was conceived naturally, but God created her soul with those virtues proper to the Mother of the Lord. But she owes all those virtues to her son whose divine nature exists before he enters her womb to assume also a human nature. We therefore speak of her as the Holy Virgin.
Personally speaking I would not use the word insemination as that is a literal term to describe what 'flesh' can accomplish. We are speaking of and attempting to describe a never to have happened before or since miracle, in as the process of the flesh body beginning without the involvement of a 'man'. IT was a miracle the conception took place.
But is NOT a new thing for to be known, some call it 'born again' when in fact from the origin of the language it is 'born from above', and according to John 3:10 Jesus answered and said unto him, "Art thou a master of Israel, and knowest not these things?
13 And NO man hath ascended up to heaven, but He That came down from heaven, even the Son of man Which is in heaven.
Mary provided the earth vessel for the flesh body of 'God with us' to come in a flesh vessel, but she had NOTHING to do with the existence and or preexistence of the Spirit that was/is and always be Christ, notice Genesis 3:15 it was already in existence.