Smallville - Season 6, Episode 16
On the day of Lex and Lana's wedding, Lex receives a call from a doctor who threatens to tell Lana the truth about her baby. Meanwhile, Lana begins to have second thoughts about marrying Lex. Clark comes to his own realization -- he can't let Lana marry Lex -- and decides to tell her his secret. But no one is prepared for what happens when Lionel steps in.