Using a multisite randomized controlled trial design , stable preterm twins ( @ sets , N = @ infants ) between @ and @ completed weeks ' gestational age , stratified by gestational age ( and > @ weeks ) and site , were randomly assigned to a co-bedding group , cared for continuously in the same incubator or crib or a standard care group , cared for in a separate incubator or crib , and underwent a medically indicated heel lance after at least @ hours and no greater than @ days of group allocation .