Try not to do anything to make things harder for them. The baby or babies may be very sick, so being too congratulatory can be unhelpful. Acknowledge what the parents are going through and their fears for their baby. If the parents have other children, do all you can to look after these children, entertain them if that's what they want, comfort them if they are anxious, provide a stress-free haven for them if you can. If you send cards or flowers, be thoughtful.
Some flowers look like they belong at a funeral.
Cards with pictures of plump full term babies can cause distress. Stock messages are often inappropriate. Blank cards on which you write your own message, poem, something from your heart, will probably be much more appropriate. Be careful not to place expectations or any sort of load on the new parents - they have a great deal to grapple with, and they may not have the time or the energy even to talk to anyone for a while. Understand if they don't call back.