Six months after Imam Hassan (AS) was born, Fatima (SA) became pregnant with her second child. Lady Fatima started noticing the signs that childbearing was near, but the Prophet (S) had already foretold of Imam Hussain’s birth.

On the third of the blessed month of Sha’ban, the fourth year after Hijra, the Messenger of Allah (S) was given news of the birth of Imam Hussein (AS). He (S) hurried to the house of Imam Ali (AS) and Fatima al-Zahra (SA).
Safia Bint Abdul Muttalib, Asma Bint Umais, and Um Salama were present when Imam Hussain was born.
When the Prophet(S) asked Safia (his aunt) to bring him the newborn child, she said: ""We have not cleaned him yet."" When the Prophet heard this, he (S) said: ""You clean him? Surely Allah the Exalted has cleaned and purified him""
Asma took the newborn to him. The infant was wrapped in a piece of cloth. The face of the Messenger of Allah (S) lit up upon seeing his grandson. He took him in his arms. He recited the call to prayer (Azan) into his right ear, and read the shorter version (iqama) in his left ear. He, then, placed the baby in his lap and wept.
""May my father and mother be your sacrifice,"" Asma asked him, ""why are you crying?""
""Because of my son,"" he(S) replied.
""He is a newborn baby,"" she said.
""O Asma,"" he (S) said, ""After me, the transgressing party will kill him. May Allah never grant them my intercession.""
Then he said: ""Asma, do not tell Fatima about this, for she has just given birth to him.""
After Imam Hussain (AS) was born, Gabriel descended to the Prophet (S) and revealed to him to give the new baby the name Al-Hussain. Al-Hussain is the Arabic version of the old Hebrew name Shabir, which was Haroun’s second son’s name. When Gabriel descended to the Prophet, scores of angels accompanied him to congratulate and console the Prophet for Hussain’s birth and expected martyrdom.
Any woman, including his mother (SA) did not nurse Imam Hussain (AS); instead he suckled the Prophet’s tongue until he grew old enough to eat. Because of this, his characteristics were exactly as those of the Prophet (S).
Seven days after the birth the, Messenger of Allah shaved Hussain’s head and gave the weight of his hair as charity for him.
(Source: Yazehra.com)