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You don't need to find expensive clothing for this project, in fact you want a plain, one-color onesie. Choose the right age/size for each progression. For example, for month two, you'll most likely need a onesie sized for a newborn from zero to three months. Consider the child's potential size. If the baby has not been born or is still very new, consider if the baby is large or small for his or her size. Premature babies typically are a few sizes smaller than their actual age, whereas some babies are born wearing clothing suited for a six month old.

Summary:
Select 12 size- or age-appropriate onesies.