New job & 4 months pregnant, when to tell

Ok, here's the situation. I'm four months pregnant and a finalist for an exciting new job with my organization. I have not disclosed my pregnancy and do not intend to (legally I do not have to, and refraining from telling actually makes fairness easier all around). My question is about what to do if I get the offer. Tell them right away or wait a couple of months to show them what a great decision and thereby softening the blow that I'll be out for a few months? I've been with this org almost 10 years so questions about leave, etc are not an issue. Thanks for any and all perspectives!