J.C.
I think that you have to give new moms a little more time than anyone else.
When I had my baby, I had post partum and although I did get out the thanks yous fairly quickly, some took a little more time. And I can recall one cousin calling to see if I got the gift she sent and I found it annoying - I was dealing with so much already. And it had nothing to do with rudeness - I always send thank yous. I just couldn't get them out quick enough.
If you got the delivery confirmation than that will have to be enough. Hopefully she will send a TY soon - and if not. what can you do. Just let it go.