virgin birth

The idea that Mary was a virgin when her son Jesus was conceived and born, making his existence a miracle caused by God. (Note: this does not mean she was always a virgin; she married Joseph and had children with him, something many Christians forget even though it's in the Bible that Joseph and Mary had several children.) Not the same as the doctrine of Immaculate Conception, which has to do with Mary's parents and was what made her a person pure enough to bear the Son of God.

The idea of a person being born without the mother ever having had sexual intercourse.

It's most often used when talking about the Christian Jesus, who was claimed to have been a miracle when given to Mary, since she was a virgin. Supposedly a gift from God.

Oddly, many people are not aware that it was common at that time for people of importance to be said to have been a virgin birth. It was also said about Julius Caesar, for example.

"(Note: this does not mean she was always a virgin; she married Joseph and had children with him, something many Christians forget even though it's in the Bible that Joseph and Mary had several children.)"

Actually, Christians have never forgotten this; but at least one denomination--the Roman Catholic Church--denies that this is the case. For starts, the Bible never says that Mary has the kids; they just show up in Jesus' adult years. Furthermore, they claim that when the Bible in its earliest forms refers to the "brothers" and "sisters" of Jesus it does so using a word that has no direct equivalent in English and which could also mean close relatives who are not immediate blood kin, such as cousins. They also raise the possibility, found in certain extra-canonical sources, that Joseph had children by a previous marriage.

I don't know if they insist that Mary remained a virgin all her life, but they do tend to take issue with those who insist that she did not.

Some scholars feel that they are reaching a bit.

According to a British geneticist who was on the Discovery Channel, a virgin birth, although it would still be a miracle, is consistent with known science!

His logic was:

1. Parthenogenesis, where a mother produces an egg with a genome identical to hers, is possible. It happens in many lower animal species, like rotifers. No male gamete is necessary.
2. It is theoretically possible for a human to produce such an egg cell, although extremely unlikely.
3. Since Mary was a female, but Jesus was male, Mary must have had a Y chromosome. This is extremely unlikely, but possible--there are women who have one X chromosome and one Y chromosome, but they develop as women. Something along the chain of what causes the presence of the Y chromosome to lead to male characteristics is broken.
4. XY women usually do not have a uterus at all, let alone a functioning one . . . but it is still possible even if extremely unlikely.
5. Jesus could have been born of a Virgin Mother who happened to be XY with a functioning reproductive system. It's still a miracle of course, because such an unlikely event was coordinated in God's plan along with other events necessary for the salvation of mankind.

Maybe we'll see this in the next Papal encyclical . . .

The Catholic belief that Mary was a perpetual virgin is related to her own Immaculate Conception and Assumption and is founded on her sinlessness. She made a vow to serve God, which included a vow of virginity. The Catholic Church contends that those who claim Mary bore children other than Jesus is a huge insult, since that implies she broke her vow. The Catholic view of Mary is that she was born free of sin, and lived free of sin, for all intents and purposes, as close to being divine as a human could be.¹

The earliest document the Church points to that supports this claim is the Protoevangelium of James. The fact that it was written shortly after Mary's death (between 120 and 160 A.D.) is used to strengthen the validity of the claim, although the four accepted gospels do not directly make the claim. The issue was brought up many times, and in the late 4th century, Helvidious wrote about several of his tenets, one of which was that Mary bore children other than Jesus. Saint Jerome refuted (successfully in the Church's eyes) this several times and further claimed that Jospeh was also a virgin so that Mary could keep her vow. Modern apologists claim that Jospeh was an old widower and was physically unable to have sex with Mary, and say his absence in Jesus' adult life (in the gospels) was a result of death, and as proof of his age. Others continue, claiming the children are in fact Joseph's from a previous marriage.

The claim that the gospels say Mary bore children other than Jesus is refuted by the Church based on a mistranslation of the Greek text. They say that the Greek word for brother, adelphos, has several meanings, citing other non-literal uses such as in Acts 3:17,22, Mt 7:3, 23:8, and Mt 25:40. It is usually noted that the Protestant founders (Luther, Calvin, etc.) believed in the perpetual virginity.

Relevant passages in the Catechism are:

510 Mary remained a virgin in conceiving her Son, a virgin in giving birth to him, a virgin in carrying him, a virgin in nursing him at her breast, always a virgin.

The gospels that may imply Jesus had siblings:

Is not this the carpenter's son? is not his mother called Mary? and his brethren, James, and Joses, and Simon, and Judas? - Matthew 13:55 (King James)

Is not this the carpenter, the son of Mary, the brother of James, and Joses, and of Juda, and Simon? and are not his sisters here with us? And they were offended at him. - Mark 6:3 (King James)

and the Catechism refutes this with:

500 The Church has always understood these passages as not referring to other children of the Virgin Mary. In fact James and Joseph, 'brothers of Jesus,' are the sons of another Mary, a disciple of Christ...

¹ I must make it very clear here that the Roman Catholic Church does not believe Mary is divine. She is most definitely human. What differentiates Mary from other humans is that she is considered the model humans must strive for, but not in the same sense that Jesus is also a model. Jesus, being divine, is capable of things Mary isn't. Mary had to be redeemed, which is the goal the Church prescribes to its followers.

important disclosure: I am not a theologian, and I am an ex-Catholic (although the Church would say once a Catholic, always a Catholic). This is based on my understanding of said documents and the Catechism of the Catholic Church.