The history of them in the US is quite different from the UK imitations.
It used to be when women gathered together to help provide for the new baby (a bit like buying house gifts for a couple who were marrying), and would have a 'woman's talk' about giving birth & being a mum. Think early pioneer days & barn raising type things.
now, of course, it's updated. but still the idea is to get gifts to help ease the cost of the new baby, and to have some female time before the new mum gets caught up in newborn baby world.
Also, in the US, it is far more the custom to have parties etc before an even, with the actual event being the kind of finale - therefore, Christmas stuff happens before & Christmas day is the final fling before returning to normality.
Def only for first borns, or if someone is expecting twins/has had a big gap & has no baby stuff etc.
Then the new parents are given some space, with meals dropped off for the first few weeks, before everyone expects to pile in & have a squeeze of the baby.