For the most part AIO's are good to go right after the first wash for normal daytime use (a change at least every two hours with a newborn.) The absorbent and waterproof parts are sewed together. Many one size AIO's have a pocket where you can add extra inserts depending on your absorbancy needs. The sized AIO's I've had experience with the newborn size did not have a place for more absorbency, but the larger sizes did.

I thought my husband would love AIO's because of the easy factor, but he actually prefers the prefolds since he can tell if they are wet easier and the washing is simpler.