What's the expected response?

My long-term – and for the last 9 years, long-distance – friend had a baby in October. I have congratulated and all that, but haven’t really been in touch much since. The other day I get an email from my sister (who lives close to my friend) saying my friend is wondering if everything is OK with me because I haven’t been in touch, even though she’d sent pictures of the little one….
What exactly was I expected to reply to the ‘look at me’ emails, except “he’s gorgeous – congrats!”?

I guess this kinda opens the other can of worms: how do you maintain your friendships of 20+ years when they’ve all gone off and had kids in the last 2 years? Is there really anything we could talk about aside from kids – which I’m not all that interested in (sure I want to see the pics and how they take after their mums and I want to know they’re healthy and happy and all that, but that’s about it)...?