Re: When to start shopping for baby.
I think it is hard to determine their size but when they leave the hospital, they’re pretty tiny and usually are wearing newborn or preemie sizes. Hospitals also provide diapers around the clock and send you home with some along with wipes, Vaseline, etc etc etc. You can use the newborn size up until they’re a month - most kids will move up after that to a 1 (just in my experience). Even if they’re big, eventually you know those diapers will get used at some point. But bulking up is a bad idea because of sensitivities.
Regarding clothes - its always best to give them bigger because they only stay in newborn sizes for a very short while. Sizes change super fast in the first year and having something on hand the next size up is sooooo helpful. I was happy to have a nice stash of 6-12 month clothes (sister gave them to me) when we were getting done with the 3-6. I did end up buying a few things anyway but it wasn’t an emergency so I took my time.
I have not heard that its bad luck though.