hypoplastic thumb - congenital hand differences - Boston Children's Hospital 

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Thumb hypoplasia, also commonly called hypoplastic thumb, means that your child's thumb is unusually small or underdeveloped. Thumb aplasia means that your child's thumb is missing altogether.
Thumb hypoplasia and aplasia are usually detected during your baby's first newborn exam.
Seeing this deformity will prompt your child's doctor to look for other deformities that are sometimes associated with this condition.
An x-ray can help doctors examine the internal structures of your child's thumb.