EDIT: Are there really people dense enough that they think that the shower is a "welcome baby" and "greet baby" party? That it's not actually about making sure that new, never-had-a-baby before parents get off on the right start so that they don't have to break the bank in buying everything babies need? When you have already had a first or second baby then you HAVE EVERYTHING and whatever you don't have you are expected to buy it yourself. Because not everything should be bought FOR you and your children. Good God. The sense of self-entitlement for ourselves and our children is staggering in this thread. Absolutely disgusting greed.
Showers are for the PARENTS and not the babies. How utterly stupid and short sighted is someone, how completely dense can you be, to say that it's "ruder" to have a celebration to greet the baby after its born. That doesn't make sense. All people want is to see and hold a new baby. People have Christenings and naming ceremonies and other important celebrations AFTER BIRTH that welcome the baby into the world and that's when you celebrate.
It's perfectly fair. What's not fair is money grubbing and gift grabbing. If you want to gather with friends for a "last hurrah" before baby comes then go ahead, but do your best to make sure that it's clear that there are no similarities to baby showers.
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First, don't ever ever ever plan your own shower or sprinkle or whatever the heck you choose to call a 2nd baby shower. Second, do not hint or get someone else to do it for you. Third, if someone chooses to throw one for you on their own just because they feel like it, then that's when you register and only then for items you might actually need.
Showers are also for parents who have significant age gaps between children AND those parents no longer have any of their old baby items and/or any items they have are not likely safe to use any longer. One other situation might be if the couple is suddenly expecting multiple babies and they need more than the one of each item they already have. Or they lost everything in a fire. Situations like that.
In your situation, a normal situation, I would refuse a shower and wouldn't register. It's presumptuous. You don't need one and anything you may need you can afford to buy yourself and don't need others to pay for it for you. If others want to buy gifts they don't need a shower for the opportunity. When the baby is born you can have a Christening or whatever ceremony it is your religion has that might be comparable, and a celebration party afterwards that also acts as a "welcome baby to the world" party. At THAT party people will either gift something or not. They'll either ask if you registered somewhere or not.
People have no sense of decorum or what's right in these situations any longer. It's not about "equal rights for babeez!" because as already stated the showers are for new parents and NOT FOR THE BABIES. Parties for the babies occur after the babies are born.