In these two clips you see Will briefly staying in prone at 5 and 7 weeks, but after a while choosing to roll back onto his back using a different combination of movements each time.

His action actions are deliberate and goal directed and show that he is able to coordinate the movements of the head, trunk and limbs to achieve a goal. The prenatal experience of turning over and doing somersaults in the womb has prepared him for this complex multi-segment control. 

Rolling over from prone to supine is achieved by creating an unstable position which produces a "falling" action of the head and trunk. Will has learned to control the momentum created by this action. 

Will at 5 weeks 

In prone, Will at 5 weeks Will is able to keep his head in the midline and lifted up off the supporting surface and has developed some ability to stabilize the trunk when moving the lower extremities. 

In the first half of the clip you see Will actively kicking, moving between bilateral hip flexion and extension. The movement of the LE's is possible because the COM is displaced cephalad with weight shifted onto the upper thorax and UE's. 

Rolling from prone to supine
After a period of kicking, Will decides that he has had enough of being in prone, and initiates rolling to supine by lifting his head, extending the thoracic spine.The neck and thoracic extension is associated with hip extension. These movements shift his COM caudally. He rotates his shoulder girdle back on the right, flexes the right hip then "falls" onto his back, displaying the beginnings of flexion balance response in the trunk and hip flexors. 

Will rolls back again, using a different strategy
I turn Will back into prone, and he immediately rolls back into supine, this time using a slightly different pattern of movement. He pushes down on his right hand, extends his elbow, and extends the thoracic spine.

The COM is shifts caudally, but there is not associated extension of the hips. Will collapses onto his left side with the hips still in flexion, and then rolls onto his back. 

Video clip: Will 5 weeks prone 
Normal speed 

Slow motion

Will at 7 weeks 

At the beginning of the clip you see Will lying with his arms in elevation, head resting on the right upper arm and his hips flexed, with small range of hip movements only.  

He lifts his head up off the supporting surface, but the neck extension is not associated with thoracic extension. 

After 20 seconds he decides that he has had enough, extends his neck and the right hip and rolls back onto his back. The rolling action is intentional and goal oriented because he repeats it each time he is turned back into prone. 

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More about Will's development at 4-8 weeks 
Prone development from 7-14 weeks 
Prone development 3-4 months