Postures of the arms in the first two postnatal months

The arm postures that infants adopt in the early months of life set up a repertoire of movement patterns that may aid in the development of reaching. There is evidence of tightly flexed arm postures in the womb, but how and when arm postures change over time after birth has not been systematically documented. The present study followed infants while lying in supine weekly from 3-weeks-old until they acquired head control. We documented the frequency rate of different steady state arm postures occurring when the hands were in contact with the body or the supporting surface. Across the observed developmental period, rates of flexed arm postures at the elbow decreased and more extended elbow arm postures increased but only around the time infants began to control their head. Initial elbow flexions, with the hands mostly oriented towards the head were superseded by elbow extensions with the hands primarily oriented towards the feet. Finally, most steady state arm postures entailed the forearm resting on the supporting surface or on the body rather than being held with the elbow in the air. Together, these findings show that arm postures adopted in the womb carry over into the early postnatal months and last for several weeks before extended arm postures become more prevalent. These findings have potential implications for the development of reaching and for preparing infants to produce arm movements away from the body.