I've never heard of such a thing.  How old is the "new" stepdaughter?  I can't imagine making a registry or throwing a baby shower for a new stepchild.  I would be embarrassed to do such a thing - seems like a gift grab.  Showers are usually held because a new mom doesn't have things for the baby and needs a lot.  I would think your stepchild already has what she needs; presumably your husband has provided for her needs to date.  Now, if you wanted to have a party to introduce your new stepchild to extended family, that's a different thing.

My parents treat my two SDs as their own grandchildren, but they didn't feel the need to announce it.  I think if I had wanted to announce my stepdaughters to extended family (I didn't feel the need), I would probably do it by sending a wedding picture that included them (they were flower girls when DH and I got married) and identifying them as his children.

ETA:  By the time I married my DH and officially became a stepmother, my SDs had been in my life for a few years.  Anyone in my extended family who would care that I was a stepmom already knew about them by that point.
 

Yeah, I'd back off on this. This isn't your child, and what kind of things could YOU possibly need as a stepparent, other than things the child's bio parents already have, or will purchase?

And it's nice that your mom is excited, but she should just wait for her annual Christmas letter to announce that you got married and there is a new step-grandkid.

Wait what? I'm confused are you expecting? Or did you just get married and your spouse has a child from a previous relationship?

If you are sharing your pregnancy announcement with your family and your spouses family do whatever makes you happy.
If you are wondering if you should share your pregnancy announcement with you spouses ex and that family...I think that would depend on the relationship.

If you are announcing to your family that your spouse has a child...I guess my question is were they not aware when you married that your spouse had a child?
You could let them know that you have "blended" and are now a SM if you feel necessary.
As for throwing a "baby shower" that's usually for parents of newborn infants who don't have any baby supplies.

A few things you need to consider:

1.) What is your relationship like with the kids? Do they see you as a motherly figure? Your mom as their grandmother?

2.) What kind of woman is the biomom? Is she open to you acting fully like a mom, not just in a parental/familial capacity?

Even though "mother" is in the title, it's really a misnomer in most cases. Unless biomom is dead, has been dead, the kids well and truly view and treat you like their mother, AND you act in all motherly - not just parental/familial - capacities, you're not a mother on the same level as a bio or adoptive mother. You're really an in-law to the kids, and we don't throw parties for new in-laws except bridal showers.

I know that sounds really cold, but you've admitted you're new to this. The dynamic is VERY different, and assuming too much about the relationship and authority you have could end up in pain and anger. The kids get to dictate if you are a "bonus mom" or just "dad's wife". You can't exceed what they're willing to allow. Additionally, overstepping your role can irritate BM, and that can cause a ripple effect of problems for your husband and the kids. 

If you want to introduce your new family to your extended family, then host a "Meet the Millers" barbecue when everyone is vaccinated. But a shower to celebrate you is ill-advised. We don't throw showers for new aunts for becoming aunts, or grandmas for becoming grandmas. You're closer to them as far as family hierarchy is concerned than Mom.

This is inappropriate to throw a shower to "support you" as a stepmom. I would be pissed if another women tried to do this for my child. Your stepdaughter is not your child. If you want to announce something, just have a family get together to introduce her.

You need to slow your roll, lower your expectations, and not do anything impulsively.

Your SO has a child with his ex, who wields tremendous power. Overstepping is the LAST thing you want to do. Blending is best done slowly and methodically, allowing relationships to grow over time. 

Coming on too strong/too fast is a common mistake SMs make, and it can cause a lot of damage. The smart move is to keep things low key, focus on your relationship with your SO, and support him in his parenting. Don't get too involved, don't overfunction for him, just enjoy being a newlywed. Don't force things, and stay in your lane.

I am confused.  Is the child this shower is for already born?  If so, how old?  

I do think this seems gift grabby, I would not do it.  I can't even imagine how a BM would act over something like this.  

Can we take a minute to be kind.  Not every step situation is a run and hide from the evil SK.  OP is new here let's get to know her before we run her off!

OP - as you can see, most people on this board take a distanced relationship with their SKs (stepkids).  Many of the posters here believe that there's only enough room in a child's life for two parents.  Any others are "additional", and should approach only carefully, and with the permission of the biological parent.  This is generally speaking good advice, from people with experience being hurt by vindictive bioMoms who don't appreciate a new "mommy" on the scene, and wimpy husbands who don't defend their new wife and their new family.  Your experience may not be the same!  Or you may want to heed these early words of wisdom.

If the stepchild is quite young, biomom kept everything, and Dad is starting off with an infant and very little to take care of said child, I see no reason why you can't throw a baby shower for Dad's house.  As you've seen above, many will get their knickers in a twist, but there's no reason a Dad can't have a shower just as equally to a Mom.  In this case, your family may want to shower you rather than Dad (that's up to you and your husband's comfort level).  To be more appropriate, it really should be Dad, but the two of you as a couple could be good too.  If the child is not an infant, or the divorce is not very very new, then presumably Dad already has everything the child needs (clothes, bed etc.).  In which case a "welcome to the family" party would be a better idea than a "shower".  Your mother sounds like a lovely lady, and I'm glad she's excited to have a grandchild.  My mother has very much enjoyed being grandmother to ALL her grandchildren for almost 20 years now - biological and step.  So don't let anyone rain on her parade.

Please slow down, for the sake of everyone involved. You and your boyfriend are moving in together, that is all that is happening. You are not getting married, and even if you were, it doesn't really change things. Presumably, he has had 50/50 custody of his daughter prior to your moving in. What gifts would you possible need in order to care for his child - which is the purpose of a baby shower.

How old is this girl? What kind of a relationship does she currently have with your Mom? If she is school age, she may not be interested in acquiring a new Grandparent - have you talked about this with her? Your moving in isn't going to magically change the kind of relationship they have.

While I admire your Mom's enthusiasm - I haven't heard of anyone in your situation doing anything bordering on a formal announcement, let alone having a shower.

Hmmm, this is new.

Not something I have ever heard of.  My SS is my parent's eldest GK.  They have been his GPs since before he was 2yo.  They did not have a "baby" shower for my wife or send out an announcement broadcasting his existence to the world.

If you marry this guy maybe mention the Skid in the marriage announcement.

Unless.... BM  and/or her mother  is/are evil in which case.... go ahead and rub their noses in it.

A wiser path is to let the relationship develop natiover time.   Your extended family will learn of the Skid as events unfold.  No need to announce to the world that your SO mated with someone else.