Signs for Knowing When to Size Up on Diapers:
One of the most obvious signs of a too-small diaper is when your little one is constantly leaking through diapers or having "blow outs." While parents may sometimes jump to the conclusion that the problem lies with the brand of diaper they're using, but in reality a diaper that is too small won't be able to contain your little ones messes, no matter what brand it is. Try sizing up and see if that eliminates the problem.
2. Read the box
This may seem obvious, but if your child is approaching the upper limit of the diaper's weight range (i.e., you have a 16-pound baby in 12- to 18-pound size diapers) it may be time to consider a switch. The weight limits are based on average-sized babies, but not all babies are shaped the same. With some diaper sizes, my daughter had to move up well before the weight limit on the box, but with other sizes (like her current size 4 diapers) she's above the weight limit by a pound or two, and they still fit fine. Try different sizes, and see what works best for your child.
3. Red marks
If your baby has red marks along their thighs, that's a telltale sign that the diapers are too snug. The elastic around the leg should have some stretch, but if the diaper is too small, it won't fit properly and will create these red marks. This is definitely a time to move up a size in diapers.
4. Check the waist
If you're having difficulty connecting the diaper tabs at your baby's waist, that could be a sign that it's time to move up a size. A diaper should close easily without having to tug and pull at them. Also, consider the rise of the diaper. A properly fitting diaper should come just slightly below your little one's belly button. If it's fitting a little more like a low rise, it's time to move on up!
By Lauren Hartmann, Disney Baby