Infants with hypermobile joints and preterm infants, who in the early months lay with there legs flat against the mattress, often develop tightness in the muscles that cross from the lumbar region over the lateral aspect of the hips, including iliotibial band. 

This tightness affects their lower limb position when they are lying prone. They often keep their hips in abduction and do not develop hip extension with the thighs in close to parallel as seen in typically developing infants.  

Below you can see the difference in the hip position between Toesies at 10 months and Will at 6 months. 

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If hip adduction is restricted the infant tends to keep the hips widely abducted when they move from sitting to kneeling at a low step. This set of pictures shows how Roan's legs  splay as she tries to get up onto her knees or kneels at a low step. . 

Hip mobilization in kneeling 
Position the infant in kneeling facing a 10cm high firm foam block with the knees about hip width apart. 

Hold the legs in place and encourage your infant to reach for and play with toys. Have a selection available so that you can change to a new game as soon as the infant gets bored. 

Video: Hip mobilization in kneeling