The great news is that you can start a baby registry at any time. Most soon to be parents will start one around 12 weeks. It may seem too early for some but you can start the registry and make it private. I would suggest to keep it private until you are sure of the items you really want and need. When it comes to baby stuff there are endless options so it can take a while for you to decide on what you will really need.
Are you planning to find out the sex of your baby? If so then you may want to wait until around 20 weeks to start your registry.
Are you planning on having a baby shower? If you are planning a baby shower then you will want your registry all set up before your invitations go out. Leave your guest plenty of time to purchase gifts.