We had our first in December. Our plan paid out according to the employee plus spouse up until the day he was born. As soon as he was born we were switched to the family plan which raised our deductible and oop rates. We had already hit our deductible and were 400 shy of the oop before he was born. Once the plan switched over, we owed about another 1500 to hit the oop amount. This will most likely happen whether you put him on your plan or your husbands, but I agree with the pp that said adding him to yours will most likely be the cheapest since you have already been paying against the amounts.
On a side note, since you are still trying you might want to pay attention to when your due date will be. I know you don't have a lot of control over it but it can make a difference. Our insurance rolls over on Jan. 1. Thankfully our DS was born the very end of Dec, but if he had been born even 2 days later it would have cost us an extra 6000 dollars. We were very lucky that the pregnancy was covered under one calendar year.