Kitāb al-Ghayba > Concealed Birth of the Imam of the Time (a.s) was not an extraordinary event
Hadith #2

وقد نطق القرآن بقصة إبراهيم عليه السلام وأن أمه ولدته خفيا وغيبته في المغارة حتى بلغ وكان من أمره ما كان. وما كان من قصة موسى عليه السلام فإن أمه ألقته في البحر خوفا عليه وإشفاقا من فرعون عليه وذلك مشهور نطق به القرآن. ومثل ذلك قصة صاحب الزمان عليه السلام سواء، فكيف يقال إن هذا خارج عن العادات؟ ومن الناس من يكون له ولد من جارية يستتر بها من زوجته برهة من الزمان حتى إذا حضرته الوفاة أقر به.

The Quran has spoken of Ibrahim and that his mother gave birth to him in secrecy and how she hid him in a cave until he grew up and then his story unraveled. It mentions the story of Musa (a.s) that his mother left him in the river fearing for his life from Firon. This famous, and the Quran has mentioned it. The story of the Master of the Age is similar to these. So how can it be said that it is against the ordinary? Some people have children from concubines that they hide from their wives, until they are at their deathbed and then they confess.