As a general note, the gospels are dated approximately 70A.D. and later – almost forty years after Jesus’ death. This is during a time without anything close to the quality of record-keeping we have today – mostly passed down orally. In my opinion, the virgin birth (as well as miracles, resurrection, etc.) is pure fiction, simply made up after the fact – maybe in an attempt to verify “prophecies” in the Old Testament, maybe to bolster a new religion, maybe for some other political or social reason. The authors of the gospels are not known to have recorded any events as they happened, and the earliest gospel (Mark – based on the memories of Peter, as well as oral and written traditions) has a mysterious author who had no contact with Jesus at all. Mark was the basis for Matthew and Luke, and there are many passages copied word-for-word. Other passages contradict, such as the day of the crucifixion (Mark and John don’t seem agree on whether or not Jesus was crucified the day before or after Passover).
The Gospels of Matthew and Luke assert that Jesus fulfilled the prophecies of Isaiah (7:14) of a virgin giving birth to a savior. First, it is interesting to note that Mark and John do not mention this fundamentally important detail. The absence of a key detail of symphonic proportions is extremely important when considering the validity of the claim in Matthew and Luke. John even refers to Jesus as the son of Joseph, and Paul refers to Jesus being born of a woman without mentioning she was a virgin.
The Hebrew word “almah” is the word used in the book of Isaiah (7:14) to describe a “young woman” or “maiden” who is of child-bearing age but has yet to have children. When the Hebrew was later translated to Greek, the word “parthenos” was substituted which means “virgin.” English translations use the word virgin. Some translations now use the term “young woman” with footnotes stating ‘or virgin’ (or visa versa) following the 1952 publication of the Revised Standard Version which used the Hebrew “young woman” instead of the later Greek translation.
Vincent Bugliosi does a much better job detailing this point in his book Divinity of Doubt. However, understanding that the original Hebrew does not support a prophecy of a virgin birth, one can argue that the prophecy and the subsequent virgin-birth story are highly suspect. It is not uncommon for changes and/or errors to be made to translations to support a biased viewpoint, and this seems a likely example.