Diapers are typically sized by weight. It's helpful to know what your baby weighs. This weight will give you a good starting point if you don't know where to begin. Start with a small package and pay attention to how the diaper fits to determine whether your baby is wearing the right size. When the diaper is fastened, you should be able to slip two fingers inside the waistband. If the diaper is too snug for two fingers, you should go up a size. Other signs that the diaper is too small include the following: Inadequate coverage over the baby's bottom. Red marks around the thighs or stomach. Moisture easily soaks through the diaper. Your baby's diaper may be too small if: You deal with frequent blowouts. The diaper sags low even when it's not full. Gaps appear at the leg openings. Your baby's diaper needs are important, but it's not always easy to find the time for a visit to the store when you run out of the necessary supplies. Through Instacart, you can order diapers, wipes, and other baby supplies easily. Have your order delivered in as little as two hours and save your busy family a trip to the store.