Salam alaikum,

shia historian al-Masoode , in his book “Isbat al-Wasiyah” p 174, wrote:

Translation:

(and it was narrated) that Fatima, alaihi salam, bore Hasan (a) in the beginning of the day and  became pregnant with Husayn in that day, because she was pure purified, and she didn’t suffer from things from which suffer women, and she was pregnant with him (Husayn) 6 month…

So what do we have here?

1) Accusations of Ali by shias. In accordance to this he was so impatient that he made intercourse with his wife in the very same day, when she bore him a child!

2) Imam Husayn was prematurely born.