This article highlights the changes in head, trunk, and limb posture that occur during the brief moments when Will is not moving vigorously from the age of 7- 31 weeks.
Over the first 6 months infants develop the ability to stabilize the head in the midline when reaching and kicking as well as to move the head to look in all directions while lying supine.
Opportunities to lie in supine on a firm flat surface provide very young infants with their first experience of linking sensory input to motor actions as a first step towards voluntary goal directed actions seen just a few weeks later.
At 5 weeks Will has developed more control of his upper and lower extremity actions and his ability to stabilize the head and trunk to support these actions has improved.
He has also learned to integrate contact with the support surface (SS) into his postural control strategies.
At 10 weeks Will's control over head and posture and movement allows him to follow a moving object across the midline. He has also learned to stabilize the trunk when kicking vicariously.
The 3-4 month period is characterized by the infant's increasing ability to steady the head and trunk in the midline,and produce goal directed, purposeful movements with the upper and lower extremities.
At 5 months Will has developed the ability to stabilize the head and trunk in supine, and shows increasing control of differentiated limb movements as he actively explores surfaces and objects with the feet and hands.
The 5-7 month period is characterized by an increasing ability adapt posture and movements to achieve a desired goal. Infant's are becoming increasingly aware of their ability to formulate goals and then initiate actions to achieve these goals.
This article highlights the changes in head, trunk, and limb posture that occur during the brief moments when Will is not moving vigorously from the age of 7- 31 weeks. 
Over the first 6 months infants develop the ability to stabilize the head in the midline when reaching and kicking as well as to move the head to look in all directions while lying supine. 
At 5 weeks Will has developed more control of his upper and lower extremity actions and his ability to stabilize the head and trunk to support these actions has improved.
Supine development: the 1-2 month period - notes and videos 
At 10 weeks Will's control over head and posture and movement allows him to follow a moving object across the midline. He has also learned to stabilize the trunk when kicking vicariously. 
The 3-4 month period is characterized by the infant's increasing ability to steady the head and trunk in the midline,and produce goal directed, purposeful movements with the upper and lower extremities. 
At 5 months Will has developed the ability to stabilize the head and trunk in supine, and shows increasing control of differentiated limb movements as he actively explores surfaces and objects with the feet and hands.
The 5-7 month period is characterized by an increasing ability adapt posture and movements to achieve a desired goal. Infant's are becoming increasingly aware of their ability to formulate goals and then initiate actions to achieve these goals. 
In this set of video clip you see Max (aged 12 weeks - CA 6 weeks) in supine and Will (full term) at 7 and 10 weeks. 
Will's development in supine from 7 -10 weeks
Increasing control of head and trunk to support limbs movements.