OK, just the clear this up because I see several people struggling with the concept: -
The "bluff" isn't whether or not she's pregnant. The bluff is her making Tyrion aware of it while simultaneously *thinking* she's attempting to hide it. She want's him to know, but she wants him to figure it out himself and she want's him to believe she doesn't really want him to know because if she flat out tells him, he'll know she has an ulterior motive.
Why is this important? Remember the context of the conversation. He's asking her what she hoped the achieve by agreeing to the meeting. What does she want out of this situation? What really matters to her in this equation? By leading him to conclude she's afraid for her unborn child, she's building the pretext that'll make him believe she's genuinely relented when she agrees to the truce. She knows that Tyrion knows that her one and only redeeming trait (in his eyes) is that she genuinely and truly loves her children and would do *anything* to protect them.
And yes, she almost certainly is pregnant.