Thanks u were a big help :-)
I was advised to make sure the corners are square and fit snuggly in crib, NOT rounded, so that baby has less crevices to get his/her arms stuck in. Also, if you can find a dual mattress (firm for infants, and softer for toddlers on the flip side), you won't have to purchase another one til he/she has grown out of toddler bed. If you already have your crib, measure it. Again, a standard double sided mattress should suffice. Don't forget super snug mattress protector and sheets!
Good luck!
Put your head to the mattres you find listen to it. Some have coils and eco everything going on. Baby will not sleep on it. good luck with your hunt
When I was registering and didn't know what brands were good etc I got on Amazon typed in the type of thing I was looking for (like "crib mattress" for you) and arranged the his by highest customer review. That made it so I could read what a lot of different ppl had to say about a lot of different things abc helped me decide.