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:
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.