Is it normal for a 1 yr old to have a curve in their back?

Scoliosis: Looking at the baby's back directly in front of you usually shows a straight spine. Any curvature to either side is abnormal. Looking at the back on either side of the baby usually shows a curvature which is often normal. Please consult your pediatrician to confirm your suspicion.

Define "curve": If a toddler has a "c" curve (kyphosis) when he sits, it can mean low muscle tone in the trunk & abdomen. If his profile looks like a lima bean in standing (lordosis), with "winging" of his scapulae (shoulder blades & "postural fixing" to perform fine motor tasks, it can also mean low core tone. An "s" curve (scoliosis) is a spine problem. His doctor can tell on physical & neurological exams.