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.
(St Augustine, Sermon 186,1)

Catholics believe that the Virgin Mary remained a virgin her entire life, even though she was married to St Joseph.

The question of her remaining a virgin during the birth of the Lord is a perplexing one. Does it mean the birth was a miraculous birth, avoiding the pangs of childbirth (which scripture tells us is a result of original sin) and avoiding breaking her virginity, or was it a natural (if painless) birth?

It is often suggested that the "brothers and sisters" of the Lord mentioned in Scripture are evidence that Mary had other children after Jesus, but:

• "Brother" is clearly used throughout scripture in the context of meaning kinsman equally as much as it can mean "brother" in the strict sense
• Hebrew and Aramaic have no word to distinguish these two relationships from each other
• The "brothers" are mentioned as "taking offence at" Jesus (Mt 6:3), hardly the kind of actions of younger siblings, it would not be their place to act in this way towards their eldest brother
• At the foot of the cross, there would have been no need for Our Lord to entrust His Mother to the care of St John if she had other children who could take care of her
• Christian tradition has always considered her to be ever-virgin

The Catechism states:

Mary is a virgin because her virginity is the sign of her faith 'unadulterated by any doubt, and of her undivided gift of herself to God's will.
(no.506)