Maybe I'm not reading this correctly ...
When is your shower scheduled to be? Is it already in the works - and you're looking for a way to word it to the guests who have already been invited? Or are you looking at a way to word a 'Meet the baby' arrival shower?
If it's the second, then I would Google ideas on Arrival showers - they are usually worded in such a way as "Celebrate our new bundle of joy (baby's name)" kind of thing.
I'm guessing though, someone has planned a baby shower for you and you're wondering how to postpone this for now. Could you ask the person who is hosting this to call the people? That's what I would do.
However, you're saying to send 'cards' so I'm not really following. You don't need to explain though, if you haven't already invited them - there's no need. I would not get into specifics. I would just hold off on sending invites - period, until after the baby arrives. Usually showers that are held after baby's arrival are about a month after the birth to give mom time to recuperate, and to make sure baby is not in newborn stage (since guests will want to visit with/or see baby).
Congratulations and I wish you a safe and happy pregnancy :)