T.H.
I think it's a little long too. And I guess I'm a little confused, I don't understand what you mean by changing of the prayer. I've never heard that term and you used it twice.
I think I would either go with the whole secret thing and skip the math part or just do the math part, which I thought was kind of cute. Although you can omit the stuff in parentheses, it's redundant.
I think it would also be cute to just have a card and on the outside put 3 + 1 = 4 then on the inside have a picture of each of you (maybe separately) with plus signs in between and the last picture be a cartoon baby, a question mark, or maybe even a sonogram picture. Keep it simple. They are going to figure it out pretty darn quick and they won't want to read through a bunch of stuff! ;)
I just whipped up this poem in about 2 minutes, so it's not perfect, but I tried to incorporate some of the cute things you had there, maybe you could make it work for you.
I have a secret I'm supposed to keep,
so don't say a peep!
My dad tried teaching me more math
but I'm not so sure I followed his path...
He said 3+1=4
but that's one more
than we have right now...so wait, Holy Cow,
I'm going to get a sister, or maybe a brother
because Mom and Dad are having another!