How did Brutus keep the secret about the conspiracy from his wife?

Conspiracies to commit murder, of their very nature, need to be kept top secret for them to work properly. Brutus has agreed to join the conspiracy to assassinate Caesar, and although he hasn't taken an oath, it's understandable that he should want to keep things under his hat. His faithful, loving wife Portia senses that something's on Brutus's mind, though she doesn't know what. She begs him to reveal what's bugging him, but Brutus, ever the Stoic, is not about to give in to an emotional plea. Even when Portia stabs herself in the thigh to prove her loyalty to Brutus, he still won't relent.
Yet, Brutus still loves and respects his wife and knows that at some point he'll need to let her in on the secret. He invokes the gods to make him worthy of such a noble wife before making Portia a solemn promise:

Portia, go in awhile. And by and by thy bosom shall partake The secrets of my heart. All my engagements I will construe to thee, All the charactery of my sad brows. Leave me with haste. (Act II, Scene i)

If it had been anyone else saying this, we might well think that they were simply trying to get rid of Portia because of a sudden knock at the door. But not the honorable Brutus. Although we never actually see Brutus telling Portia about the forthcoming conspiracy, it's clear from her own words and behavior that she does indeed know about it:

Yes, bring me word, boy, if thy lord look well, For he went sickly fourth. And take good note what Caesar doth, what suitors press to him. (Act II, Scene iv)

The Ides of March have arrived and Portia is in a frantic state, her behavior consistent with knowing the details of what's about to happen. In the above quotation she's instructing Lucius to dash off to the Capitol to find out what Caesar's doing, how he's doing, and who's near him. As soon as Lucius takes off, the soothsayer enters and Portia starts getting even more agitated, desperate to know if he prophesises any harm coming to Caesar. It's unlikely in the extreme that Portia would act this way if she didn't know what was going on. It's a measure of how much Brutus trusts his wife that he feels able to share with her such a terrible secret.