Any and all of the above!
I have searched high and low in Hallmark stores, the Caligraphy card and gift stores, and in overseas card shops where they seem to have every card imaginable (eg. 'congrats to my sister-in-law for expecting', 'happy 27th to my brother-in-law', 'Congratulations to my uncle on his first baby'...) but haven't seen anything beyond 'thankyou' cards which aren't really suitable.
Looks like I'll just have to make my own.