I sometimes answer, "Probably not - we're not saints yet!"
Other responses, "We just had this one, so we're not expecting another one *right now*."
Depending on my mood, "Maybe ... it was a difficult pregnancy, and I'm really tired!"
Or on a good day, "Are you kidding? Look how perfect he is!"
Since our religion does not permit sterilization (which is what, "Are you done?" questions imply), our feelings about having a larger family, on any given day, don't make any difference. What matters is the long-term commitment to be open to new life in our marriage.
I make the assumption that people asking about our family size are just curious, rather than hostile, or maybe not even curious, just not able to think of anything to say, after, "Wow, your baby is fat!" (At least the next line isn't, "And so are you!") I try to make it clear that we don't believe it's entirely up to us whether we have more children. We could decide we have enough, and then have another anyway, or we could decide we want more, but not be able to have them. Either way, the final decision is God's.
Congratulations on your daughter, and best wishes to you and Mrs. G-b for the remainder of the pregnancy!
Congratulations and God bless.
Thanks TC - and you are right. The sterilization 'suggestion' is embedded. We don't forsee taking that road for a long, long time, if ever.