The Heidelberg Catechism, This Lord's Day week 14

Q35: What is the meaning of "conceived by the Holy Ghost, born of the virgin Mary"?

A35: That the eternal Son of God, who is [1] and continues true and eternal God,[2] took upon Himself the very nature of man, of the flesh and blood of the virgin Mary,[3] by the operation of the Holy Ghost;[4] so that He might also be the true seed of David,[5] like unto His brethren in all things,[6] except for sin.[7]
1. John 1:1; Rom. 1:3-42. Rom. 9:53. Gal. 4:4; John 1:144. Matt. 1:18-20; Luke 1:355. Psa. 132:116. Phil. 2:77. Heb. 4:15: I John 5:20

Q36: What benefit do you receive from the holy conception and birth of Christ?

A36: That He is our Mediator,[1] and with His innocence and perfect holiness covers, in the sight of God, my sin, wherein I was conceived.[2]
1. Heb. 2:16-172. Psa. 32:1; I John 1:9